John Watson is endlessly fascinating in a way that wouldn’t show were he to donate blood/skin/hair/saliva/semen for close study. That doesn’t mean Sherlock will stop asking, if only to watch him turn delightful colours at the question. For a doctor and an army man he is oddly reluctant to ejaculate into a specimen jar.
Then, of course, he is hardly standard issue in any context. Because he is a doctor, regular army issue, but he has that same quiet capacity for sure-handed violence that Sherlock has seen in veterans of less visible wars. He makes tea and he giggles and he once shot a serial killer in the back of the head then called Sherlock an idiot and ate char siu bao with his fingers. Brilliant.
John calls him ‘friend’ when he is happy and ‘colleague’ when he is annoyed and, on one memorable occasion, ‘stupid thoughtless arsing bastard cunt’ when Sherlock had stepped neatly into the path of an incoming bullet. (It was sheer commonsense, he continues to argue long after the scarring fades. The height disparity meant the choice between a broken collarbone or an additional hole in John’s occasionally useful face.)
The time between John moving in to the Baker St flat and John becoming a necessary part of Sherlock’s world isn’t entirely quantifiable, although Sherlock does admit to being a less than objective project participant. The most interesting experiment of his entire life, as it turns out, is an indelible part of that life. He can no more claim scientific distance than those unbearable cooing pseudo-scientists who claim they can converse with chimps.
When he shares this thought with John, in a rare moment of camaraderie, John huffs up to his room and refuses to let Sherlock borrow his laptop for a week (incomprehensible man). It was a compliment.