Sometimes I think a very large part of writing is figuring out my characters' motivations. Why do my characters do what they do? Yes, it has to do with what they want and who and what stops them from getting what they want. And it also has to do with how they see themselves and how that changes when they get involved in the plot.
Being true to the motivation for all characters--even villains who, naturally, see themselves as the hero of their story-- can take you a long way down the narrative path.
But this motivation question is not just about the big events. Every scene, every gesture, every conversation and silence, has motivation in it and can, if done right, reveal character. Every little thing done by every character has to be accounted for. And when you have main characters who are on stage in a scene and act in ways that feel inauthentic, it is usually the unfortunate failure of motivation that is behind their inauthenticity.
Why do my characters do what they do in both small and large ways? I try to keep coming back to this question. A lot of discovering and revealing the secrets of character lies in motivation.