I often have the same problems and as i go through the 2/3rds of my complete book I'm trying to correct this problem and flesh out the characters alot more. I think first off it's important to know where the character will be at the beggining and end of the book and what they will go through. This is important because as you and I change constantly in our lives, so should your characters. I try to make it so you can tell the differences in characters throughout the story so you grow to understand the character and why and how they are evolving the way they do.
To do this, (and I am still trying to fully figure this out although I many never fully comprehend it) use the interactions between characters and in the case of omnicent naration, the feelings of your character in the face of adverse situation. Also, body language is 80% of communication. Don't neglect this in your writing simply because it is writing. Don't forget your characters should move and act in certain ways that reveal emotions as well and help to realize your characters personalities.
The idea of modeling characters after real life people can be a good one, but can often lead to problems when put in these very different situations. I could not tell you exactly how the people I know, or even myself would act facing life and death situations. I know how I would like to think I would act, however human nature allows us to think a little better of ourselves than is actually true.
So basically, find out who your characters are, why they are who they are, what drives them to act the way they do, and why and how they change through the story, then, implement that through actions, reactions, dialouge and naration. That would be my advice, and I will try to take it in my writing as well!
thanks for the thoughtful topic and I hope something in this mess of a reply helps