What?
A character interview is pretty much what you might imagine: a creative exercise in which you ask your character questions and "listen" for and record their answers. It's an exercise in letting go of what you *think* you know about your characters and story and letting your subconscious mind (which, at least in my case, is WAY more creative and awesome than my conscious mind) do the talking.
Why?
There are SO MANY REASONS you'd want to do character interviews, dear fiction writers. By which I mean there are at least seven.
1. Create conflict. You can use character interviews while you're brainstorming/plotting to help you get to know them and the conflicts they have - both within themselves and with each other. This can lead to clearer thinking around: (a) what your characters want and how they need to develop over the course of your story in order to get it, (b) how they might react in certain situations because of their triggers, and (c) what kinds of challenges will create productive conflict in the book that helps them grow as well as moves the plot along.