All good, round, deep characters have a flaw. Sometimes it's a tragic flaw; sometimes, it's a fatal flaw. Other times, it's the shattered facet that keeps a character human.
So what's a flaw?
Often, it's a lack of something.
If a character feels a lack of something, it motivates him. Often, nearly always, this character has a goal that symbolizes, to him/her, the satisfaction of this inner, private need.
It could be love (too many to mention), success, money (all the Nero Wolfes), affection for his family (It's a Wonderful Life,) , a family, safety from a criminal (Silence of the Lambs), a certain level of society (The Great Gatsby,) etc.
In short, it is anything that threatens your character's sense that s/he controls his/her own destiny.
When a person is aware of such a lack, they are ill at ease. They try to compensate. More on compensation tomorrow.