Filling In the Back Story

I tried to work on Chapter 5 last night and failed miserably. I was so frustrated because I just couldn't figure out a way to get started. What really pissed me off was that I couldn't figure out why I couldn't figure out how to get started. This morning I finally figured out why I was stuck. Chapter 5 starts out with two of the characters that just aren't that well developed in my mind yet.

I know who the heroine is. I've known her for months now. Sure, she's changed a bit here and there as the story has evolved, but I know who she is - how she'll react in any situation. Same with the hero. He was harder, as I'm not a guy and can't necessarily get inside his head as well, but I know where he came from and I know where he's going.

The other characters aren't as well developed in my head yet. So before I can start Chapter 5, I need to fill in some of the back story on a few of the other characters. Once I finish that, I think I'll be on good shape to kick Chapter 5 out pretty quickly. I know what needs to happen, I just need to figure out how.