Characters are supposed to have motivations for their actions.
Gee, isn't that obvious?
Apparently not to moi. Several contest entries and critiques have pointed out the lack of motivation on the part of my characters. Ouch. I spent so much time taking apart my words and putting them back together that I didn't focus on making sure my character had reasons for doing things.
This is very important. Like House says, everybody lies. And everyone has a reason. So do my characters. And unlike real people, they should be consistent.
I'm thinking that in a story it's good to have one main thing that drives your character. Perhaps justice. Or truth. Or maybe something more concrete. Like needing a new job.
I guess it goes back to goals. In your story (and mine) each character should have both internal and external goals. And if the story is a romance, well, then the hero's goals should clash with the heroine's.
For me, a seat of the pants writer, this sounds very difficult to think about. I prefer to sit down and plop out whatever comes to mind. This means lots of revisions later. Ugh. So I'm trying to think my plot through before I write it.
Another cool thing to do with your plot is layer it. For more on that, you can check out the Craftie Ladies of Suspense.