Simply obvious to most but I don't think I ever did this at AM. Let your character breathe, even if it's subtle, even if it's not so evident in the dialogue, even if they didn't just finish running a marathon. Let's try to find places to incorporate a sense of inhaling and exhaling. Chest expansion, nostril flares, shape change on the lips,etc. And it's more than just energy expenditure right? Like the idea that someone might start to hyperventilate when they're nervous. Play it in degress, appropriate to the action. If I neglect to consider this in my initial blocking, I'll always make sure (and by "always", I mean I've done this one time, on my last shot, two weeks ago) , I'll make sure to do a "breath pass" before I get too far.
Here at Blue Sky, without singling out any one animator, I would hear that, "With His/Her scenes, the characters are just...ALIVE!" I would scour the animation tests that were being done for the characters before I came on board. I picked out a simple dialogue test, and among other things, I noticed that the character's chest would expand and contract right where you'd expect, and I freaked out! It was suuuper subtle too. I was like, "Man I'm gonna do that!" So I did! I did exactly that on my shot. Since it was all pantomime, I blocked in the my acting, then went in and looked for opportunities to put in breaths. Varied them up a little. And when I showed the Anim. director my blocking, he called it out! Swear to God he's like, " I like that breath he takes right there , that's good thinking." Which felt pretty nice because I'm usually not too smart about these things. This whole thinking thing.
It makes a world of difference, and people might not notice it but they'll feel it. And it's one of those things that'll help you inject more life into your characters.