James N. Frey, author of a couple really great advice-about-writing books that I love (How to Write a Da*n Good Mystery and The Key: How to Write Da*n Good Fiction Using the Power of Myth) recommends doing journal entries as the main characters in your stories. Basically, letting them say some stuff in their own voice about who they are, what they're up to, why they're doing stuff, etc. This to me is an incredibly brilliant and helpful idea, and I don't know why nobody else ever suggested it to me before. I've been trying it for the main characters in my novel recently, and it's working great!
When I wrote my first mystery (utilizing the advice from Frey's book on mystery-writing)--my Angel and Combat! crossover, "Searching"--I found journaling-as-characters very helpful. Especially for Angel himself, because I just couldn't seem to write him. When writing my fanfic, I often have trouble capturing my favorite character from the show (Sgt. Saunders in C!, and Angel from his show); I think my adulation of the characters gets in the way. Anyway, I was having a really hard time figuring out Angel's attitude toward the whole situation in that story. He kept not wanting to talk or do much of anything--was being very passive and grumpy. Broodiness, I'm used to from him. Even reticence. But inactivity? I hadn't expected that from him. So I did a journal entry for him. Let him just write about what he was feeling at the moment, what was going on in his head. And it did the trick! I discovered what was holding him back, and figured out what would get him back in the game.
If this sounds a bit mystical to you...if you can't quite dig the idea of getting into a fictional character's mind and writing a journal entry (or anything else) as them...I'm not sure I can explain it (altho if you insist, I could try). But it works!