A/N: For Mr_CSI, on the joys and sorrows of role-playing sex with one's flatmate. I really just wanted to see if it was possible for me to write anything in the canon!Sherlock!verse, other than AUs. I'm expecting this to be around 6-8 chapters, and I feel like I might be updating faster than I do my AUs, but I still wouldn't recommend the holding of breath.

"Fantastic," Sherlock breathed.

He could see John rolling his eyes behind him out of the corner of his own and he smiled slightly. He liked it when John did things like that – it signified the things that he had accepted about Sherlock that had driven all his predecessors away. He had never expected to find someone who merely smiled encouragingly at him as he peered intently underneath the fingernails of a dead body.

The dead body, in this particular instance, was stretched out on a magnificent four-poster bed with limbs flung to the four posts, looking like a cross between a starfish and a gruesome, slightly overweight caricature of Jesus Christ on the cross. He was mostly dressed, his crisp white shirt unbuttoned to expose the soft swell of his belly and the pectoral muscles of someone who used to exercise religiously but has let themselves go, the equally expensive black slacks unbuttoned and unzipped, the soft, pathetic-looking penis hanging out over the fabric.

He pulled out his magnifier; there were strips of soft, flexible tape – marketed as bondage tape, Sherlock guessed, pulling on it to test the elasticity – hanging limply from the posts at the corners of the bed. Someone had been tied to this bed, and recently, judging by the amount of stick on the tape. But there were no obvious ligature marks on the dead man's wrists, and a quick examination under the magnifier showed that there was no trace of the hair loss that would surely result from ripping off the tape.

He'd been strangled, though, with no signs of a struggle. He had seen murders much like this before, and yet he would have expected the victim to be the one who had been tied down, a submissive who had picked the wrong person to dominate them. This was very unusual. It was wonderful.

John made a noise of half-hearted protest as Sherlock used his latex-covered hands to lift the man's penis and examine it. Sherlock turned away from it briefly in order to raise an eyebrow at his friend, the non-verbal how else am I going to learn flitting between them until John made a tiny gesture as if to throw his hands up in surrender and turned away.

There were traces of dried semen around the tip. The killer had definitely taken satisfaction from the man somehow before strangling him.

"Not so petit mort," he commented drily, removing the gloves with a snap. John grunted in acknowledgment of the joke. Lestrade looked puzzled. Sherlock sighed. "He was killed post-coitus," he explained impatiently. "Evidently before he had had a chance to tidy himself up."

The DI grunted in sympathy. "He was into being tied up, though," he mused, pointing at the ends of the bondage tape. "You'd think he'd choose his partners more wisely."

Sherlock tutted. "The victim wasn't the one who was tied up," he said, impatience leaking through his voice at the way in which the Yard insisted on dawdling so far behind his deductions. John had that delightful little half-smile on his face that said he knew the reasoning behind this one. "If the killer had tied him up and killed him, why bother untying him? And the victim's wrists show no signs of having been bound – no ligature marks or depilation."

Anderson cleared his throat pointedly from the doorway. "Would you let the forensic scientists take a look at it, then?" he said nastily. "If the killer was the one tied up, there might be DNA on the tape."

"No," Sherlock rebutted instantly. "The tape's much too short. He or she would have been cut free and taken the bits of tape still on their wrists with them. Pay attention, Anderson."

The scientist rolled his eyes despairingly at Lestrade. The DI, however, wasn't looking at him, but staring thoughtfully at the body. "She," he said quietly.

Sherlock looked at him. "What?"

"The killer was a woman," Lestrade explained, flicking the file in his hand open and looking at the papers inside it. "We interviewed his neighbours when we first got here and they said they were quite loud, and they could definitely hear a woman's voice. They also gave us the name of the bar he usually goes to, Sally's there now talking to the bartender."

Sherlock nodded. "You told her not to bias the witness by being hostile, didn't you?" he quipped. Lestrade's fed up expression coaxed a smile onto his face.

"She won't bias the witness," the DI said in a long-suffering manner. "Sergeant Donovan is a perfectly capable police officer, despite her personal flaws." Sherlock raised an indifferent eyebrow and let the issue rest. He didn't like Donovan, but he did have to admit that she was a passable officer. Lestrade grinned at him. "Go on, then," he said, sighing as though it was the last thing he wanted.

Sherlock quirked a smile. "Even you can tell that he's wealthy. Probably arrogant, too, judging by the way the wealth is displayed throughout the house and the clothes he's almost wearing. So – young, passably handsome, rich and arrogant man who's dominant with his sexuality. I would hazard a guess that he's not particularly careful with his submissive lovers. There must be a long list of women he's abused under the guise of BDSM."

Lestrade sighed. "Can we narrow down the list in any way?" he asked resignedly. Anderson made his usual noise of disgust at the ease with which his DI trusted "The Freak". Sherlock glanced at him scornfully.

"It's probably recent," he mused, peering at the victim's exposed belly and groin again. "I mean, they're probably a recent conquest of his. But I doubt he would have shared his sex life with his neighbours or his friends." He frowned down at the man. "If the barman saw them you might have a physical description. Who knows, maybe he even knew her." Lestrade's face took on that look that it had when he knew there was nothing else he could do. Sherlock shrugged. "Send me the case file and the barman's statement and if the barman doesn't know who she is John and I will do some investigations."

John snorted. "What, are we going to embed ourselves in the S&M scene?" he asked playfully.

Sherlock shot him a grin and muttered, "You wish." He hadn't meant it to be flirtatious, but from the way the doctor grinned back he could tell it had turned out that way anyway. Sometimes he wasn't sure whether John returned his flirting because he thought Sherlock only did it to unnerve the Yarders or because he, like Sherlock, did it without realising, did it because it was the natural response to the way John looked and spoke and moved.

"All right, you two, you can't shag here, it's a crime scene," Lestrade muttered, making shooing movements with his hands. John started on his customary we're not a couple before giving up. Sherlock was never sure whether he should be pleased that the doctor seemed to think better of his protests or upset that he still attempted to make them.

The flirting that had passed between them was forgotten by the time they made it back into a taxi. Sherlock quite often wondered if he was imagining it.

Lestrade's text arrived as they were getting out of the cab at Baker Street. Sherlock made a noise of interest. John looked up eagerly, as though ready to get right back into the car. "The barman didn't know her, but he was watching them for a while. Got a good description of her, and of the way they were behaving."

"Oh?" John asked, holding the front door open for Sherlock to pass him. "That's good."

Sherlock hummed happily. "I'll look at the statement inside – he says it looks like he took control from her. Could be interesting."

John frowned as he hung his coat over the hook by the door. "I can't imagine what that would be like," he said slowly. "To trust someone with that kind of intimacy and then know in your last moments that they betrayed you."

Sherlock snorted. "If you want to trust someone with that, don't pick a stranger," he rejoined. It was so easy for him to imagine John doing it, finding someone good-looking who was willing to indulge a kink that sent others running, so eager to be affectionate that he would trust them without knowing more about them than their first name. He wasn't willing to examine the flicker of jealous anger that the thought sparked in him. John just trusted people. Sometimes it worried him and he wanted to beat the habit out of him. Other times he worried that if he did, perhaps John would no longer trust him so implicitly.

"Unfortunately, if you don't pick a stranger, you won't get anyone," John said wistfully. He accepted Sherlock's coat when it was held out to him with an absent expression, as though he didn't notice he was doing it. That was another thing that Sherlock valued so much about John: Sherlock had become such an intrinsic part of his life that it was as if the doctor attended to his every need without thinking about it, on automatic pilot the same way in which he brushed his own teeth. "And sometimes you need someone."

He thought about rebutting the point. Everything he needed from someone he got from John. Everything else was something that people wanted. He had to admit - only to himself, of course - that he sometimes thought about seeking those things from John, too. Only in an abstract sort of a way, in that sometimes when he pleasured himself the doctor's face popped into his head, in that occasionally out of curiosity he wondered whether John would like to hold or be held, whether he himself would rather have his friend's strong arms around him or his stocky body held tightly against his own chest. He shrugged instead, affecting disinterest, and wandered off to find John's laptop.

John wiggled a mug of tea slightly beside his head so that Sherlock could hear the no doubt perfectly prepared liquid sloshing around inside it. He reached up and took it without looking up from the laptop screen. "Thank you," he said absently. Lestrade's eyewitness statement from the bartender had just arrived.

The doctor hummed. "Where's your laptop? If you're using mine, I'm going to use yours."

Sherlock finally looked up at him. "It's in the bedroom," he said, noticing the tiny flush that always made itself known on John's face when he said the bedroom instead of my bedroom. "But it doesn't work properly anymore, I took out half the RAM to use in an experiment."

John blinked at him for a moment. Then he sighed in slow-motion, an elongated breath in and a pause and an elongated breath out. "Fine," he said calmly. "I'll check the blog in the morning, then."

"Don't go anywhere, though, John, I'll need you in a minute," Sherlock interrupted as the doctor began to move off. He waved an arm at the laptop. "Lestrade just sent through the eyewitness statement of the victim and that woman at the bar. I want to go through it, see if their behaviour gives away any clues as to why and how she killed him."

The doctor folded his arms, frowning curiously. "Why do you need me for that? If I'm just filling in for the skull again -"

Filling in for the skull had become a sort of code between them, John's way of calmly letting Sherlock know he was using him, inconveniencing him when he didn't really need him there. Until John had started pointing it out, Sherlock hadn't realised how often he kept John around simply to watch his fond, amazed smile. "No," Sherlock replied hastily. "I want to re-enact it. To get an idea of what he would have been like, what they would have been like together. What he might have made her feel."

John stared at him. Sherlock realised how the sentence had sounded after a moment of his gaze. "No, I mean - what he might have done that annoyed her, how she might have picked him out as someone that she wanted to murder. Seeing someone act the way he might have could help."

"Oh, right," John said, nodding. "Okay. Let me get my tea."

Sherlock smiled; he'd known John would agree, even though pretending to pick each other up at a bar definitely wasn't something that John's idea of 'normal flatmates' did. He brought up this notion of 'normal' when he was angry with Sherlock, but he never seemed to adhere to it himself. He was pretty sure 'normal flatmates' didn't sit quite as close to one another on the sofa in the evenings as they did, or have quite so few boundaries when it came to sharing the bathroom. Sherlock himself didn't particularly care whether John saw him urinating or showering, and he attributed the doctor's corresponding lack of shyness to his time in the army and to his medical disregard for nudity. "Don't bother, we'll do it in the kitchen," he replied. "They were standing at the bar; the kitchen counter's the closest thing we have."

John nodded briskly and set off for the kitchen. Sherlock smiled fondly after him. He skimmed his eyes over the rest of the statement – the eyewitness they had interviewed had eventually stopped watching because the pair had seemed to forget they were in public once they started kissing. The dominance of the victim was already clear from the kiss, though. A tiny shiver announced itself up Sherlock's spine, and he followed John into the kitchen.

"She approached him," he opened, setting the laptop and his untouched cup of tea on the kitchen counter between them. John turned up his chin in that stoic way he had when he had set his mind on doing something. "Suggests that she'd already picked him out from the crowd – maybe she followed him there, or maybe he just happened to make the wrong gesture at the wrong time. Either way, it's hardly the behaviour of a full-time submissive."

The doctor snorted. "I think murdering someone is hardly the behaviour of a full-time submissive."

Sherlock tipped his head in acknowledgment, grinning. "Perhaps not," he agreed.

John returned the grin. "So, I'm the victim," he said abruptly, leaning against the counter, holding his mug of tea much in the same way Sherlock had seen him hold a pint-glass. He neglected to point out that the victim had been drinking spirits. "Approach me."

His tone of voice was one Sherlock had never heard directed at him before: bold, seductive. He gave his friend a wry grin before stepping candidly right into his personal space. "Good evening," he murmured, toying with the idea of putting a woman's high pitch into his voice before pitching it low and sultry instead.

John's entire demeanour changed. He stood up straighter, no longer resting the majority of his weight against the bar. "Evening," he replied, a tiny note of pleased surprise in his voice, which had dropped half an octave to reflect Sherlock's own. There was a tiny comfortable pause, and then John said – in the same hypnotic tone of voice – "Do we know what they said to each other?"

Sherlock smiled seductively, reaching for his mug of tea and letting his fingers play suggestively with the handle before picking it up. "The witness was on the other side of the bar," he replied, keeping up the act. He wanted to know how John, who knew people's character even if he was not an expert on deducing the small things, thought the victim would talk and stand and move. Exactly what he was saying wasn't important. "But I don't think they talked for very long." He dropped his eyes from John's with a reluctance that wasn't entirely faked and checked the report. "No," he said, angling the screen towards the doctor and using the gesture to mask a tiny step closer to him. He'd never been on the receiving end of John's seduction before. Was this a technique he used with the women he dated, or was it something he was making up just for the role-play? "The real intention was always clear, talking wasn't necessary."

John laughed, a low rumble that really wasn't too different from his normal laugh. He pushed off the counter and put down his tea, the predatory expression on his face giving way to something almost tender for a moment. "And then what happened?" he asked, his voice quieter now, more intimate.

"She reached up and touched his face," Sherlock continued, doing so, the slightly rough skin of John's cheek warm under his hand. The doctor's face was slightly flushed, his thin lips parted. He looked, beautifully, like a confident man who had been absolutely blindsided by unexpected attraction to the person in front of him. Sherlock wondered why he'd never thought much of John's acting skills before now.

He leaned in slightly, and John copied him as though the movement was subconscious, involuntary. A smile wormed its way onto Sherlock's mouth; John's eyes followed the movement helplessly as he leaned in further, his own lips parted, expectant. Sherlock could feel the power, the electricity, the intoxicating knowledge that he had John, and a tiny flicker of sympathy for the murderer he was impersonating crept up on him.

John's breath blew across his face, smelling of tea and peppermint toothpaste. For a tiny instant, Sherlock considered actually kissing him and passing it off as a part of the role-play. He'd wondered for a while what it would be like to kiss John's lips. But the doctor would be kissing him and pretending to be someone else, so it wouldn't be the same.

"And then they kissed," Sherlock breathed instead, his voice coming out quieter than he had meant it, more intimate, as he tried to prevent himself from breathing on John's face. He had had coffee when John had had tea, and his own mouth tasted slightly sour.

John let out a long breath, his blue eyes fluttering closed. Sherlock swallowed. He knew he ought to move away from his friend, call off the role-play. He'd caught the intended glimpse of motive, and he knew John wouldn't be comfortable with the actual kissing part of the account. But the space between them now felt comfortable somehow, just more evidence of how much they trusted each other, and Sherlock rather liked the reminder, liked being close enough to kiss but not needing to.

"How did they kiss?" John whispered unexpectedly, his eyes still gently shut, his Adam's apple bobbing as he swallowed. The words tickled Sherlock's face as they went past.

Sherlock's eyes slid sideways, glancing at the laptop screen, even though he'd memorised the statement already. "Violently," he answered, suddenly not enough breath in his lungs. "He took control, pushed her up against the bar. And that's when the witness -"

He broke off with a surprised gasp as John surged forwards, their closed mouths bumping painfully together, John's body marshalling his own backwards until the small of his back hit the lip of the kitchen counter. He was momentarily surrounded by John's breath, warm and close, as the doctor's hands held him firmly around the waist and his closed mouth pressed insistently against Sherlock's, demanding and controlling. Sherlock made a helpless noise, feeling his face flush red as soon as the sound escaped. It was just as he had always imagined kissing John would be like, in the rare quiet moments when he had allowed himself to imagine: unrelenting, focused, commanding. It was lucky the kitchen counter was holding him up, because his knees had buckled, and they were only bumping their faces together like teenagers.

John pulled away as suddenly as he had advanced, breathing hard but steadily and smiling his usual bold grin. "Like that?" he asked.

Sherlock had to clear his throat before he answered. "I believe so," he replied. "The witness stopped watching them after that out of embarrassment."

The doctor nodded. "Right. Well, is that all? I might take my tea up to bed, if it's all the same to you. I'm covering Tamsin's early shift at the surgery tomorrow."

And when Sherlock nodded he turned around and marched out of the kitchen, and the sound of his footsteps on the stairs followed almost immediately. Sherlock didn't trust himself to move away from where he was still leaning against the bench, the pressure of John's lips and nose and chin lingering against his own. He reached up a helpless hand to his lips and wondered whether he hadn't made a mistake beginning this.

Fuck.