Ask them stupid trivial things, just for the fun of it. Like favorite and least favorite food and color. It might not add to their over all character much, but if you keep it stored somewhere, it might be nice little reference you can get in there whenever they decide to eat something.
Of course, you should always be willing to CHANGE these answers. Never have stuff not stated in the plot set in stone. If it can change and the plot would do better for it, change it.