To be prepared for hallway snapshots, practice a couple of poses and facial expressions, and stay in character. That'll help "sell it" to a casual observer, or someone who only sees that one shot.
For a more extensive shoot, you can definitely do a range of expressions, both in and out of character. Even a "stoic" character is going to have a handful of expressions and head/face poses that will work and remain within canon. For example, a quick Google Images search for Mai turns up a pretty good range - none of them smiling, but you've still got a variety. Expressionless/serene; scowling/disapproving; angry (nose pointed down, eyes looking up past the brow); aloof (nose pointed up, eyes looking down the nose at the camera); attacking (mouth open, or gritting teeth, or grimacing); the over-the-shoulder glare of death.
...and, of course, the out-of-character outtakes and bloopers are often the most fun shots of a shoot.