Without even interacting with someone we read their body language, facial expression, where their eyes are and we can deduce (to a degree) what kind of day that person has had or even what sort of indivdual that person is. As a good friend of mine aptly put, if he doesn't want someone to sit next to him on the bus he doesn't need to say a word. He can close off himself by folding his arms, make himself more dominant by taking up more space than he needs (gotta love the nature channel) and he can look them dead in the eye as to show his dominance even more. I know what you're thinking, but he's actually a super nice guy:P
It's amazing that we can tell the difference between just a look and a "I'm in love with you"- look, that we can feel safe walking up to one person and terrified of another without having met either of them! Where this becomes even more interesting and certainly relevant to animation (I'm slowly learning that nothing isn't) is that this language of the body can be unique to a particular character.
I for instance used to scratch my thumb when I was nervous, or tense up my stomach muscles the same way people generally hold their breath. And when I get agitated my head starts to itch.
Taking note of all these things can open up a whole new dimension, one that shows what's going on in the real world and not just the world of words. Here's a fun example from The Office where Jim gets jinxed by Pam. The only background you need is that Jim is in love with her, and the last bit is wonderful and real.
"You say it best
When you say nothing at aaall"