Had a character in my WIP who would not let me pin her down. She's evil, but that's all I could see in her. Did I want her to appear to be self-absorbed, slightly dumb, or tooth-aching sweet? Should she know she's evil when all is said and done, or should she suffer under the all-too-common murderer's delusion that she did what she had to do? I tried all the usual methods, letting her talk, letting others talk about her, but in the end I went ahead and finished the story without a clear picture of her. And yes, that was hard to do.
But it worked. Once the story was complete and I knew what had to happen, I saw how her character should affect things and how she could (and will a few days from now) hide her evil ways from others. I know it's backward and it shouldn't succeed, but what I've found in this mess we call writing is that whatever works, works.