Sherlock Holmes isn't sure about John Watson right away. That in and of itself is unusual, as he typically forms instantaneous, and negative, opinions about everyone he meets. The truth is, Sherlock is so engaged in his work when Stamford brings John to meet that him that he fails to register his presence almost entirely until John offers him the use of his phone.

It's a nice gesture, and of course plenty of people are nice, that doesn't esteem them one bit in Sherlock's mind. But it brings John to Sherlock's notice, and Sherlock finds him a fascinating collection of data and, moreover, not immediately detestable. Which is an improvement over everyone else he's considered sharing his flat with. He might be tolerable. More than tolerable, he might be actually useful.

Might be, he's ordinary, almost extraordinarily ordinary like he's trying very hard to be as ordinary as possible, but there's the tan on the back of his neck, and his ears perk up when there's a sudden noise and he always looks you directly in the eye, even when your eyes are darting about like they're scanning a computer screen that's fixed to the inside of your skull, which tends to put people off…

That's a new thought, and Sherlock likes new things. There are so few of them. The flatmate idea was a mere experiment and it hadn't occurred to him that he might find one that was both good company and competent to help him with his work. They'll have to do something about that limp, though.

Sherlock Holmes is 75% sure about John Watson when he asks him if he wants to see some terrible things and the man all but bolts out the door after him. This number slowly rises when John is openly admiring of his deduction skills (80%), tells his brother to go hang (85%), comes at the call of danger (90%), and manages to keep up on a rather hair-raising foot chase through the darkened streets of London (95%).

The way John had stood up to Mycroft, really quite impressive, Sherlock would pay a lot of money to have been able to watch that, not a shred of fear in him or a moment of hesitation in refusing to spy on Sherlock, pity he didn't know about the bugs Mycroft was constantly planting in the flat, it had really just been a test of his brother's new companion, but brilliant response none the less…

When John Watson calmly shoots and kills a man for threatening Sherlock's life and then cheerfully needles Sherlock about his stupidity for it only minutes later, happily agreeing to a late dinner out after all that, Sherlock Holmes is surer of him than he ever has been of anything in his life.

When Sherlock first realizes his complete and utter dependence on John, it's a bit of a shock.

It happened so slowly like when you put a frog in cold water and turn up the heat gradually and it doesn't notice it's being boiled alive until it's far too late…

At first living with him is mainly a novelty, but Sherlock comes to find the regularity and precision of his activities (when they are not being interrupted by Sherlock himself) fascinating, and almost as soothing as a the ticking of a clock. When left to his own devices, John rises at approximately the same hour everyday, he makes tea and toast and reads the paper. He goes out, he does chores, he watches telly, he goes to bed. It's all very consistent and, somehow, pleasant. Sherlock enjoys the background of hum of John's simple routine almost as much as he enjoys disrupting it.

He's a soldier, he likes structure and order, can't stand the messes Sherlock leaves everywhere except Sherlock's own bedroom, but won't say a word because he also likes chaos and danger and the two things manage to coexist in 221B as long as they are both there and that's why John feels so at home, routine and mundane constantly being blown to shreds by cases and experiments like in a war zone but with biscuits…

It benefits Sherlock in more direct ways as well, having someone about to keep food in the house, make tea, do laundry, and generally remind him to eat and sleep. John seems to like having someone to take care of, though he grumbles about it and Sherlock's spectacular laziness when not intrigued by a case. Which is patently ridiculous. Sherlock isn't lazy at all. He works very hard, but only on things that deserve it. The decomposition rate of human fingers do; the dusting does not.

Having John with him on cases is invaluable, although Sherlock would never admit it. The combination of medical expertise, military training, unswerving loyalty, and bravery bordering on the suicidal is mind-bogglingly convenient. John's well-developed sense of morals is handy to consult, and infinitely superior than having to keep some in his own head all the time – it's like having a portable conscience he can ditch when it gets annoying. A one-man army in a hideous jumper.

Not to mention the praise, the admiration combined with the gentle ribbing and attempts to deflate Sherlock's ego which manage somehow to feel like still more praise, he's a constant audience which is what Sherlock needs more than air and never grudging in his acknowledgement or resentful or jealous like so many others, just full of quiet amazement sometimes hidden behind cheeky criticism, but never very well, it's better than cocaine…

The benefit of John that Sherlock least expected, however, is his sheer likability, friendliness, and ability to function among normal people and do a decent impression of one of them. People like John. Pretty much everyone likes John. People don't like Sherlock, and they tend not trust an arrogant, unlikable man with no friends, no matter how many times he ends up being right. But if John likes Sherlock, maybe Sherlock is okay after all. Sure, he's eccentric, but he must be all right if John trusts him.

Of course John's the furthest thing from normal, well, not as far as Sherlock but almost and in a different direction, and he's learned to hide it and only bring it out when it's needed and Sherlock should be taking lessons from him, because deceiving plainness like that could be very helpful indeed…

It's amazing, the difference in the way Sherlock is treated when John is around, even if he's just standing there, small and unassuming, with his hands folded behind his back, taking things in quietly. John apologizes for Sherlock, John gets people to give him things they wouldn't otherwise, John warns him when he's being Not Good. John makes the whole confusingly loud and jarring world of social interactions so much more simple, and when he finds himself deferring to John for the third time in a day on how to respond to another human being, he realises what has happened.

He's found someone he simply can't do without. It's enough to make his blood run cold. He doesn't know if he can conceal it. John is more observant than Sherlock likes to give him credit for, and has already begun to figure out things about Sherlock most people never do.

"One day," John remarks casually in the taxi back home from the scene of a particularly vicious double homicide, "You will have to tell me exactly what it is you did to Sally."

"Excuse me?" Sherlock is deeply surprised, both by the comment and the fact that John has managed to arrive at such a conclusion.

Why can John always surprise him, he's dull and predictable but Sherlock's never sure what he's going to do next, even when he's plotted out his most likely moves and words with such accuracy that John sometimes wonders if the man can read his mind, but then he goes and reads Sherlock's instead and it's all very startling…

"What on earth makes you think I did anything to her?"

"Her sheer level of abhorrence for you. It's quite strong. And highly specific."

"Anderson abhors me as much if not more. Think I ate his mother?"

John laughs. "Anderson is a terrible person who hates anything he doesn't understand, which is pretty much everything, and is afraid of people who are smarter than him, which is pretty much everyone. Whereas Sally Donovan is actually a decent human being, and, in general, quite nice. She only hates you. So, what did you do to her?"

Sherlock is impressed, despite himself. It's a decent deduction and, unfortunately, an accurate one. He's not particularly proud of the incident, but John deserves his cookie for figuring it out. He likes to encourage higher level reasoning in his friend, even when it becomes personally awkward.

John's been learning, he's been watching him, he'll have to be very careful now, he knows Sherlock's methods and is using them on him and starting to know him very well indeed, and Sherlock isn't sure if being known is quite such a good idea but it seems too late to stop it now…

Sherlock sighs theatrically. "Very well. Quite a few years back, before I began my official association with Scotland Yard and Sally was in a very different department, I encountered her during a private investigation of a string of antique thefts. She was just starting out in the force, and I thought she would be the easiest officer to talk into giving me information on their lead suspect. So I used my talents to flatter her…"

"Oh God, Charming Sherlock," John says, as if that is a distinct individual. "Charming Sherlock should be illegal. With the dimples and the cheekbones and the voice. You can make straight men go weak at the knees and lesbians re-file their paperwork under hetero with Charming Sherlock. It worked, I assume?"

If Sherlock were a weaker man, he would have blushed at this frank description of his highly-effective sex appeal, but he is not and does not.

John's seen him turn it on and off like a light switch and it disturbs him, particularly when he pours out his tears like a faucet and then yanks the spigot shut again, and he wonders how John can ever believe anything he says after those performances, or can he always tell if Sherlock is shamming him or not because he never lets on if he can…

"Of course it worked. I got the information, then she asked me back to her place. She's never been short on confidence."

"So…what happened?" John asks, suspicion starting to edge into his tone.

Sherlock has the good grace to look at least a little ashamed of himself. "Well, as far as women went I had never…" He clears his throat. "It seemed like a nice opportunity for an experiment, as she made it clear she only wanted a one-night stand."

"An experiment. Christ. So basically, she went to bed with Charming Sherlock and woke up with Sociopathic Sherlock." John clearly disapproves.

"Not…exactly…" Sherlock admits with reluctance. "The illusion became a bit harder to maintain as things progressed, and the experiment quickly proved exactly why women had never entered into my activities and it all got a bit…not good…rather fast. And then I noticed a couple of case files on her kitchen table and scribbled some notes for her on them, which was not well received either. Honestly, I never expected to see her again, but then she got assigned to Lestrade's division and…here we are."

John puts two fingers to the bridge of his nose, his worse-than-I-thought expression. "So, in summation, the problem is not that you fucked Sally as Charming Sherlock and then didn't call her back. The problem is that you seduced Sally as Charming Sherlock, turned into Sociopathic Sherlock halfway through, completely failed to fuck her, and then showed her up professionally on your way out the door. Correct?"

"To be fair, she said some rather unkind things in the moment…" Sherlock says weakly.

John gives him a look. He's disappointed, and it makes Sherlock defensive.

He hadn't meant to hurt her, she'd wanted a bit of fun and as long as he gave it to her did it matter that it was just an experiment to him, it wasn't like she'd expected to settle down in Surrey and have babies and it wasn't his fault it hadn't gone right, he'd had incomplete data and solving her cases was meant as an apology, not to throw it in her face…

"Well, you wanted to know."

"I suppose I did," John admits with a rueful smile. "So girls are definitely out for you, then?"

"Married to my work, John. Remember?"

"Ah, of course. How could I forget?"

John winks at him, and Sherlock gets the feeling that John does not believe him for an instant. Frankly, he's starting to have doubts himself.

Because somehow John and his work had become the same thing and it was starting to get confusing in his head, because he couldn't do the work without John any longer, so what did that make John and what did that make him and did it really matter anymore so long as the work got done and John didn't leave…