"I'm going to find your secret," Sherlock tells him on a down day, when the boredom is creeping in and he needs a distraction. John looks up absently from his medical journal. "Pardon?"
There's always something. It's the one rule Sherlock has grudgingly learned to live with. There's always something, one detail just a bit off that he always gets wrong. It's only one, which for everyone else on the planet would be more than an accomplishment, but Sherlock has always been better and that one thing drives him mad.
Sherlock watches for any tightening of the muscles, any instinctive twitch or flicker of the eyes. He sees the slight curling of the hands- but it's such a broad movement that could mean anything. He needs more data.
It's probably good, that flaw. If he could read everyone exactly right, where would the challenge be in that? The problem arises when after months of living together he still cannot divine the missing piece. No mystery has ever persisted that long while in such close proximity to Sherlock.
"Your secret," Sherlock repeats. "Whatever is so important you find it necessary to construct an entire life to protect it. I'm going to find it."
The problem with John Adams is Sherlock can't find it. He's too simple to read, too… ah! Something. He's stereotypical but extremely idiosyncratic. He's emulating bits and pieces of so many people at once, (including himself? Probable) cobbling together his own personality from those fragments so to be unrecognizable for the mimic he is. But Sherlock knows it's an act, because Sherlock has guessed everything else right – and since he cannot have every fact right, he therefore can assume a majority if not all of his information is incorrect. However, this leads to a new deduction, possibly the one clue to the true personality of his flatmate.
John smiles – an open invitation for Sherlock to try. "We all have secrets." He says, keeping to ambiguity. It's no fun if Sherlock has clues. "And if there's one thing you're good at, Sherlock, it's uncovering them. The only question is, how long will it take you?" He settles back into his seat, relaxing his hands around the journal, and like that the teasing shadow Sherlock has been given of the man under John Adams disappears.
John Adams is a superb actor, and like Sherlock he loves a good challenge.
"I'm going to the store, later. Shall I pick up anything for you?"