I think that writers are naturally observers. We drink in the world around us.
For example, I noticed that when the clerk at the convenience store talks, none of her teeth show. And I noticed that a co-worker shuffles when he walks.
Of course, being a writer, I file all this great stuff away for later use. Here are some recent observations about people in my world:
--picks at cuticles and makes them bleed
--always chewing gum
--wears ring on thumb
--scrapes bottom lip with front teeth
--voice drops at end of sentences
What about you? Do you collect mannerisms and movements like baseball cards, filing them away until you need them?