Because both of the main characters in my current novel are mentally ill. The people they interact with the the worst is themselves.

Oooooh, ouch. I like this answer a lot.

I will try to flesh out side characters while they're in the spotlight but that's just to make the world fleshed out. I don't let them take over the story (anymore--that was a learned skill, there!)

It's tough, isn't it, that balance between making sure the supporting characters aren't cardboard cutouts and that they don't get so much detail they take over the story... But it's a fun challenge, too!