You're just about to fall asleep when you hear soft violin music drifting from the common room you share only with James Potter. It is precisely because of this fact that you think the music can't possibly be live – it has to be from a recording, because James Potter playing the violin? Not in this life. You find it odd that he'd even be listening to violin music, and therefore you come to the conclusion that James must have fallen asleep in the common room with the radio on. You check your watch: at 11:30pm, it's an entirely plausible theory.

You turn on your side, try to fall asleep, to fall back into that comfortable cocoon of almost-sleep you were nesting in, but now that you've heard it once, the music pushes into every part of you, and as you begin to discern the melody you realise the piece is familiar to you, so familiar; it's almost embedded in your spine, and though it's been years, your fingers start searching out piano keys on the mattress below you.

There's no point even trying to go back to sleep any more. The music is in you, and you feel a desperate need to go to it, to find the source of it, to share it with people around you, even if the only candidate does happen to be James Potter. In your mind, he's still asleep on the sofa with the radio accidentally turned to the late-night classical station – and that's why, once you've put on thick woolly socks and padded into the common room, you're met with a shock that makes you stop dead.

James Potter is standing in the middle of the floor in front of the fireplace, playing a beautiful mahogany violin from memory. His dark hair is falling over his closed eyes, and he's swaying on the spot as his bow caresses the strings with the kind of ease that can only come from years of practice – the fluidity of the movements reminds you of your own fingers flying over the piano keys, in a manner they haven't for many years now, and you briefly wonder why that is, why you haven't so much as sat at a piano bench since fourth year.

You stand there in awe for a moment longer, until you can't take it any more and you awkwardly clear your throat. The music stops abruptly, and he turns, the hand clasping his bow dropping slowly to his side.

"Um, hi," he says, setting the bow on a table and running his hand through his hair. It flops down over his face immediately, but you can see his eyes through the dark curtain, and they're staring straight at you. "I, uh, thought you were asleep," he continues.

"Almost," you say softly. "Then I heard the music. It... it was beautiful, James."

He smiles self-consciously. "Thanks, I guess," he says.

"It made me want to play again," you say with a nostalgic sigh.

He raises his eyebrows. "You play?"

"Played, past tense. Piano. For, I don't know, ten years almost?"

"Well," James says. "Would never have pegged you for that type. Why did you stop?"

"I..." you hesitate. You've never discussed it with anyone before, and never in your wildest dreams did you imagine that James Potter would be the one you took the topic up with, but it seems like you've caught him in a moment where he's less Potter and more James. You take a deep breath and go on.

"My mother... she was my inspiration for playing, really. When I was little, she would play in the evenings when I was falling asleep – she made sure I grew up with music. She was the one I'd go to every time I learnt something new I wanted to show someone, she was the one who would sit with me and go over a single bar or phrase for even hours on end if I couldn't get it right. In my head, I suppose I equated her with music... so when she died, I just... stopped. It didn't seem right any more. It was like music was nothing without her. But now, you, and this piece... for the first time since fourth year, I want to go back to a piano. I don't even know if I can play any more, but I just..."

James smiles gently. "It's okay, Lily. Don't be afraid of the music." He sets down his violin and walks over to the sofa and adjacent table. "These will have to do..." With a wand motion that makes it all look simple, he transfigures the sofa into a baby grand piano and the table into a piano bench. He looks up and smiles. "It's all yours, Lily. Go ahead."

You stare at the piano, unable to speak. Your fingers are itching to just run over the black-and-white keys, and you feel a familiar tension in your calf muscle as you visualise pressing down on the pedals, but something in the sight of the piano still puts you off, as if it's telling you, no, Lily, this is bad, this represents bad things, this was a bad time, don't come back to this. Deep down, however, you're aware that the piano wasn't the bad time, the bad time came when the music went away, but in your head it's always been one and the same.

"Do you want me to go away?" James asks quietly.

"No," you tell him, "please stay." Hesitantly, you take a step towards the piano, and another, until you seat yourself on the soft velvet of the bench. You place your hands on the keys, warily, and feel your fingers almost tingle at the comforting feel of the plastic-coated wood. You look at your hands resting on the keys as if they belong there for a few moments more, and tentatively start to search out scales, as familiar to you as the fingers picking out the notes. With each sound it gets easier, and you feel more and more at home with the instrument again, until you suddenly start playing the piece James was playing earlier; even though you hadn't even thought about it for three years before hearing James play it, the music flows from somewhere within you like you'd never stopped.

After some time, you hear James join in with his violin. He jumps in at exactly the right moment, playing boldly, with none of the reserve you're exhibiting, and through him your strokes grow more determined, you pick up the pace infinitesimally and gain confidence with each passing beat.

The piece builds into a stunning crescendo, and the two of you are playing almost as one. Then, without warning, you hit the final note. As you slowly release the pedal and your last note dies out in harmony with that of James's violin, there is sudden silence. You look up and see James looking at you with an inexplicable expression of fierce pride, and after a few seconds of staring straight at him you burst into abrupt tears.

He's at your side within moments. He sits on the edge of the piano bench and warily puts his arms around you. You turn to cling to him, his hand rubs your back, and through your tears you look up and whisper, "thank you." You press your face into his shoulder, content to merely be comforted for a while, and as you close your eyes and begin to drift towards sleep, he presses a soft kiss on your temple.