It's not intentional, not the first time.
Sherlock's reading the paper, sprawled out in the chair in a way that looks horrifically uncomfortable, muttering to himself about whatever he seems to think is most moronic today.
John sighs and leans over the back of the chair, hand wrapped round the leather so he can read over his shoulder. Because Sherlock rarely notices, or cares, and the paper doesn't tend to stick around long. It'll either be screwed up in a huff, or torn into pieces, or set fire to in some sort experiment. John lets his fingertips rest against the back of Sherlock's neck, fidgeting restlessly, while he reads a story about bad street lighting. He manages three more stories before he realises that Sherlock hasn't turned a page. Usually he's more reckless and abusive in his page turning and he certainly reads faster than John does. Today he doesn't actually seem to be reading at all. Instead he's holding himself very still, head tipped to the side just slightly under - under the mindless drift of John's fingers.
John thinks about snatching them away. It's a knee-jerk reaction because he doesn't - they don't - not like that. But John has never seen Sherlock indulge in anything. At least not anything that didn't involve murder or cryptography.
After a pause, in which his brain is mostly curiousity and madness, he very carefully moves his knuckles, one rolling glide down the back of Sherlock's neck.
There are no words to that, just a quiet gasp, and there's absolutely no way of mistaking exactly what sort of gasp it is. What sort of indulgence this obviously is. John has to wonder what on earth he's doing, and why he hasn't stopped yet. Why he is, in fact, now shifting his fingers, slowly, but unmistakably, intentionally, against the back of Sherlock's neck. Swallowing roughly under that rush of strange, helpless arousal that comes when you know you're doing something you really shouldn’t. When you're doing something you never expected to find yourself doing. But it's suddenly impossible to stop.
He risks turning his hand round, fingertips gliding up and then down the length of Sherlock's neck.
Sherlock inhales, sharply through his nose, and for a fraction of a second John thinks he's going to pull away, or tell him to stop. But then his head very slowly tilts forward. Far enough that John can get his fingers down the back of Sherlock's shirt collar. Where the skin is warmer and impossibly smooth. He can hear Sherlock's rough, shaky swallow, and the way air rushes out every time his thumb trails the hairline.
The paper eventually slithers out of Sherlock's grip entirely and hits the floor in pieces.
John turns his hand, fingers sliding up into Sherlock's hair, pushing it the wrong way and Sherlock tips his head back into the pressure. His fingers are now curled round the metal of the chair's arms so hard they've gone white. It occurs to John that he's watching Sherlock crack in a way that's honest, and absolutely human. A way he was fairly sure he'd never see and it's - he wants to say fascinating but the hard thumping on his own heartbeat and the unsteady and unexpected weight of his own arousal tells him it's more personal than that.
He's afraid to speak, he's half afraid to breathe.
"Do you want me to stop?" John asks and is astonished at how rough his voice is.
"If you do I will kill you." Sherlock sounds half-drunk.
John manages a broken, startled breath of laughter.
He's low enough that his knee's nearly on the carpet, so he decides 'to hell with it' and settles there. One hand curled round the back of Sherlock's neck and the other - the other dares to tug his collar aside and slide inside. Working on instinct more than common sense - and still more than a little shocked at his own daring.
"Jesus, Sherlock." His voice is a breathless shake with more than a little blame in it. Like this is just another thing that Sherlock has made him do.
Sherlock's shirt is tight and there's almost no room for John to slip his hand in, to lay his fingers on skin, which is warmer than he's expecting, smooth and flat and suddenly far too real. The shirt buttons protest in sharp creases of cloth. But Sherlock curves back into the chair, shoulders shifting to give him room.
"Sherlock," John says again, into the curve of his ear. It's half desperate, trying to find some sort of reassurance that this isn't a huge fucking mistake. Some sort of reassurance that this is ok.
His fingers spread on Sherlock's chest, his smallest finger dragging over Sherlock's nipple and one of Sherlock's shoes slides across the floor in a rush of sound. John does it again, just to hear the ragged bitten-off gasp. John can't reach any lower, can't get past the third button, but Sherlock has his head pressed into John's so hard it almost hurts, and there are no words anymore. John's other hand is pushed up into his hair, tangling there too tightly while he breathes warmth into the shell of his ear and Sherlock shudders like there is nothing else in the entire world he'd rather be doing right this second.
One of Sherlock’s hands leaves the chair arm, the movement quick and jerky like he can't help himself. He curls it over his head, fingers tangling through John's too-short hair.
John doesn't stop, locked helplessly in this strangely intimate moment that feels indecent and claustrophobic and more than half-mad. Too aroused himself to do the sensible thing - no longer sure what the sensible thing even is. He turns his head, swears, opens his mouth on the back of Sherlock's neck and bites down.
Sherlock goes completely taut in the chair, makes a noise that's half-strangled.
John's still breathing too fast into his hair when Sherlock relaxes completely, slides down in the chair on a thready little half-sigh, head rocking back to rest against John's shoulder.
They're more than close enough for John to see what he's done to him. Sherlock's breathing slow and rough, eyes half closed, the skin up his throat and on his cheeks is flushed pink.