Sherlock bides his time, as the cleverest predators do. He keeps up the façade of hesitancy until John has stepped forward, attention on the door. He waits after until the handle has been turned, until the door has been pushed open, just so, just there, just...

It's probably twice as awful that he's watching very closely, with the utmost fascination, the faint flutter of the pulse in John's neck when he shoots his hands forward to dig his fingertips sharply into John's ribs and gives a little bark of a shout. Probably twice as awful that he's watching so very closely for every last visible tightening of muscles, every last detail.

Why he feels a need to confirm that John has an autonomic nervous system, he doesn't know. Obviously he'll be as capable of startling as anyone else; he's human. But then as always knowing is an entirely different thing from experiencing, and Sherlock only regrets that he can't slow down time to watch it all moment by moment, impulse by impulse.

It's just a game. Just the sort of trick boys play on one another. That's a good enough cover, he supposes, for the underlying things, the far naughtier things.

It's just a game, not indulgence; that's all John needs to know. And Sherlock has no doubt he'll get him back, besides.