(Co-written with a friend; their part (the fluffy first part) is in italics. Mine is the worse bit.)

John's eyes flickered up from his dreary paper as the melodic, sad notes flowed off of Sherlock's violin. Sherlock stood with his back to John, his lean figure elegantly moving in slow, sad circles with his violin outstretched in his arms. The golden morning light curtained around his body, sending long shadows across the room. John felt mesmerised by the he played it with such ease, twisting and turning. The violin sang a sad song, and John almost found himself sad just to hear it.
Sherlock abruptly stopped playing, letting his arm drop and ceasing the song clumsily. John frowned, and quickly had the sense to avert his eyes back to his paper before he turned around.
Sherlock let the violin rest on his neck. He turned slightly towards John his eyes fixed on him as he tried to figure him out.
"John," He began, squinting in the light. "Do you play?"
John frowned. "Instrument wise?"
Sherlock nodded vaguely. He smiled slightly as John met his gaze and lowered his instrument.
John shook his head. He had once played the piano when he was very young, if you could call banging the keys and playing the most obnoxious little tunes 'playing'.
"No," John answered simply. "No. Not since I was a kid."
Sherlock drummed his fingers on the wooden surface of the violin. His eyes flicked away back to the window, releasing his piecing gaze from John's. John closed his eyes to regain himself and looked back down at his newspaper, not really seeing the words.
"Have you ever played the violin?" Sherlock inquired, twisting around with his hands behind his back and striding towards John. John had always been amazed by how he could take such large yet elegant strides.
John shook his head.
"I've never been that musical." He justified. "Why'd ask?"
Sherlock stood in front of John, blocking the light so he was just a tall silhouette. He bent down and handed John the instrument, before standing back, as if waiting for excellence. John carefully gazed down at the violin in his hands. This- this piece of excellence- was Sherlock's violin. The very violin that he had rehearsed, composed, and even just improvised on.
"Why, why have you given me this?" John asked.
Sherlock smiled and stood bolt upright with his hands behind his back. He looked at John like a cat would look at its prey- interested in what it was going to do, but knowing it would not succeed anyway.
"I want you to play it."
John scoffed, carefully placing the violin on the coffee table. He, of all people, would not do Sherlock's violin injustice by trying to randomly pluck notes on it. This violin was too important.
"I'm serious, John." Sherlock said sternly, his smile disappearing. "I want you to play it."
"No, Sherlock," John sighed, slightly frustrated. "I'm not going to do that. This violin has given beautiful tunes, and I'm not going do it an injustice."
John's eyes averted back to his paper, and when he looked back up again Sherlock was standing right in front of him, trusting the violin in his face.
"John, you are going to play this damn violin."

Blinking a few times as he tried to look away from that heart-stopping gaze, John gulped and sighed. "I can't Sherlock…" he whispered.

"Then I'll make you," Sherlock promised. John looked up, and realised he was rubbing the smooth strings with his index finger, longingly, subconsciously. Sherlock smiled and raised his eyebrows, as if to make it even clearer that he'd noticed.

"Of course…" John muttered, embarrassing himself again, but there was no time for that. Sherlock dropped to his knees in front of him, stroking the insides of his thighs with the tips of his fingers. John's breath quivered and he found himself unable to look away as Sherlock nuzzled him gently, watching his reactions all the while.

"Come on, John…" the man teased as he felt John begin to twitch. "Just play the violin."

And then he stood up, leaving his jumper-wearing lover dazed and distracted and desperate. "Sherlcock- SHERLOCK ahhh, I, um…" he stammered, and closed his eyes. "You… you can't just do that."

"I'm Sherlock Holmes. I do what I want." And he winked and Watson trembled with arousal, biting his lip hard. "Well?"

John had no choice. It was either play the violin or play with himself. And Sherlock was so good… It was mind-blowing, the way he used his tongue and his hands in such sexual elegance that it left John begging for more. With Sherlock John was always scared, always vulnerable, always overpowered – and that was just the way he liked it. He eyed the violin, propped it under his chin clumsily, and glanced to Sherlock who nodded that he was doing it right, and as a reward he took off his scarf and tossed it to the floor with a tempting smirk.

"Keep going, and so will I."

John gasped a breath, and tried not to look. The ultimate goal was that he did this well and Sherlock kept up his part of the deal. Don't look, touch later, John told himself. He fiddled with the tuning and plucked nervously, feeling like he was ruining it, but with a tip of Sherlock's head buttons popped open with each string's sound. It was like being given sex for good work. John's leg twitched. He hoped he could actually play something before-

Don't think, just do it, he cursed, and grabbed for the bow. He was even distracted by the brilliantly shiny screw at the end – what he longed for most of all. Stop thinking!

Sherlock's royal purple shirt drifted to the ground, and his belt slid from his trousers. As Sherlock inhaled he became so incredibly thin that John couldn't look away, his muscles perfectly formed and his hips thin and sunken.

"Aww fuck…" Watson panted.

"Easy, doctor," Sherlock taunted, licking his lips sensually. "This will only hurt for a second." His voice was raw and dark, like he was speaking in shadows, luring John in with each word from his sweet lips…

John couldn't stop himself and moaned quietly, much to Sherlock's pleasure who dropped his belt to the ground with a hard thump, making John flinch again.

But he had to continue, for his own sake. He raised the bow like a sword and attacked the violin, which roared a fantastic tune as man battled with music. It was stunning – the way he played expressed such passion, such emotion, such… lust, and Sherlock found himself weak at the knees as though John was the one in control. He tore down his trousers as he told him, "Keep playing…"

John laughed loudly, with relief and pure joy, but faltered as Sherlock climbed into the chair with him, lying across his lap and clinging to him, nipping at his neck quickly and hungrily and the tempo accelerated as with John's pulse. Sherlock slipped his hand under John's striped jumper and pinched his nipple and John lurched forward, causing the note to exaggerate and yelp.

Sherlock bit his lip but his breathing was laboured all the same as the blood rushed from his head in the most sensational way, draining his thoughts so he could focus on one thing – John. He leaned himself into him, drifting with the excitement of John's song, unzipping the maestro's flies and slipping his hand inside his boxers.

The sound paused for a few seconds besides the sounds of the men panting and gazing at each other.

"Keep going," they both said at once, and the climax of the tune approached quicker than either would have imagined. It was wild and alluring and alive with sex, and Sherlock's hand was as quick as John's bow, strong and well played.

But all of a sudden Sherlock grabbed the violin from John and tossed it carelessly to the ground, where it pinged a last note dejectedly. John seemed puzzled for a second but the mood soon shifted when he found Sherlock's tongue in his mouth. He passed out for a brief moment, recovering with head spinning to find Sherlock hadn't even taken a breath.

It was like…

It was like John was in control.

As he looked down he bit his lip and realised that his partner was fully undressed.

John was in control.

With a bite at Sherlock's tongue, sharp and quick, John pushed him back and off of him. It was the first time he ever saw Sherlock intimidated. He looked down at him and grinned, his eyes narrowed, and as he stood his trousers slumped at his feet. Sherlock tried to back away across the floor and finally pulled himself up onto the sofa, squirming.

"John, stop this," he said deadpan.

John shook his head, smiling evilly. "No, Sherlock. I finally have leverage."

And he picked up the violin.

Sherlock gulped.

He wriggled deeper into the seats of the sofa, his face tingling with embarrassment at his situation. There was John - jumper-boy for God's sake! - standing over him, playing that sweet seductive symphony slowly on his violin. He bit his lip, weakened by the music and touched to his very core. Tears welled in the corners of his eyes - tears of pride, love, and desperate arousal.
The odd thing is... he observed. I have no idea what to do next... Me. Sherlock Holmes, consulting detective. The man who... who...
He never finished the thought as John looked dead into his eyes, freezing his mind as he realised how dark and dilated Watson's pupils were: it was like he could see into John's deepest desires, and he knew that included him (and jam). Still playing, the violin and with Sherlock's delicate sexuality, he stepped closer, eyes hungrily taking in all of the exposed man before him.
The song slowed to a lullaby.
"Sherlock," he sang softly, perfectly in tune to the violin, "Just close your eyes, take a breath, listen to my voice..."
Slowly, shivering all the while, Sherlock began to let himself drift into a daze, eyes closed, breathing deeply, hearing every tiny syllable of John's melodious words. John giggled silently, blushing slightly, as he watched the dark-haired warrior lean back across the sofa.
"I'm here, my love..." he whispered, letting the notes trickle of the bow as he leaned closer. He set the violin down carefully, and as quietly as possible he undressed himself and crawled on top of Sherlock. He placed his hands on his chest and felt his heartbeat - increasing as he moved closer and closer until-
Sherlock's eyes flickered open a second before John kissed him, touching their tongue delicately together, and then more ravenously until they were tight against each other's bodies, pressing into one another, moaning with pleasure at the pressure between them. John ran his legs along Sherlock's, hands caressing his waist as Sherlock held him closer, arms tightly wrapped around his chest and back.
John clutched at Sherlock and they both drew a sharp breath, Sherlock flinching weakly, unsure whether to beg for him to stop or continue - Sherlock was not the vulnerable one. And yet... and yet...
John slid himself against Sherlock while running his hand quickly, unable to stop a few of his own moans escaping as he heard Sherlock panting.
"John..." he murmured. "Dear God, Johhhhn..."
The texture of his voice was silken with sex and yet frantic. John knew that this was his only chance to be in control of the great animal that was Sherlock Holmes, and damn it all if he wasn't going to take advantage of that.
"Yes..."
John bit his lip hard and worked harder until Sherlock was unable to control himself and moaned loudly - the masculine moan of pure pleasure that would turn almost any man. Their bodies shook together rhythmically, until, finally, in perfect harmony, they released themselves and stopped jerking and hugged one another tightly, waiting for the panting to pass.
They kissed quickly, sweetly, until finally they snuggled together and let their eyes drift shut.
"John?" Sherlock managed, with a slight giggle.
"Yes, Sherlock?" John replied nervously, already worried about what was ahead.
Sherlock rested his lips on John's ear. "You'll pay for that."