According to Neill Hicks, author of Screenwriting 101, a writer shouldn't fall back on describing their characters just in terms of hair color, height, occupation, etc. Although these traits are important, far more important is what your characters DO. What your character does is what moves story -- not their hair color, height or occupation.
On the same note, Hicks, also states that if story is what a character does, then it can also be WHAT THEY DON'T DO.
Characters don't instinctively make dramatic decisions. Like everyday humans, characters take the minimum action necessary so as not to risk betrayal of their internal need [Ed. "flaw"].Just like us regular folks, characters will almost always deny any call to action, even if taking that action is exactly what they need to become more complete, better-functioning human beings. Let's face it: all of us are afraid of change. Change disrupts our lives. This is why we so often prefer what is uncomfortable as long as it's something we're used to (therefore not disruptive). Kind of like staying in a relationship that doesn't work anymore because we don't want to rock the boat... I've heard it described once as sitting in a puddle of pee; it smells bad but at least it's warm and cozy.
It is thus the screenwriter's job to push their characters into action in a way that is credible. Your characters don't want to act, but you must make them do so, as their actions are what creates story.