Holmes is the one for whom it seems like a real strain to keep his feelings under wraps, while Watson is very practised at not reacting to Holmes’ deductions and jibes. It’s clear that Holmes isn’t her first client as she’s brisk and cool towards him without being unkind, in a way that suggests to me that she’s had to deal with a lot worse from previous addicts. She’s unruffled by things like Holmes effectively telling her she needs to get laid — which, by the way, I am so sure that as an attractive but unfriendly/uncompromising woman she’s had to put up with from hundreds of other guys already. — Hello, Tailor: Elementary 1x02.
I didn’t have room to add more in the original post because I’d already written SO MUCH, but seriously, I fucking lovethe way 90% of Joan Watson’s characterisation boils down to her having had it up to here, already. It’s pretty easy to pick up on the ways in which Elementary Holmes differs from classic asshole-genius character like House, Sherlock, etc, but I think at least part of that is down to the fact that Joan Watson just wouldn’t put up with that shit. She knows precisely where to draw the line, and when it’s drawn, that’s where shit stops.
We don’t know much about Joan’s backstory yet because she isn’t exactly a sharer, but a) her current job involves her moving in with recovering addicts and making sure they stay on track, and b) she’s a petite, attractive Asian woman who was a surgeon 10+ years, ie 10+ years hanging out with the egotistical divas of the medical profession. So it’s not hugely difficult to hazard a guess as to where why she developed her awesomely no-nonsense attitude. For Wilson in “House, MD”, it’s kind of exciting to have an exasperating buddy who acts out all the time and ignores social convention in the name genius. Joan Watson, on the other hand, will take the genius part and tell Holmes to leave the rest of it at the door.