What you really need to build a character is exactly what you don't have in the movies - time. You know, movies are like a line drawing. You have to make very quick decisions, which are, in the end instinctive. Or you make a decision to say "Well, maybe I can do that, because... Oh, that could be irritating after a while, or distracting, etc. etc." Some of it is a matter of time, always.
