It’s a good idea to write down all you know about characters and to flesh out you who want them to be, but don’t try to do too much at first.  Chances are you’ll dump most of your ideas, as clearly shown by the first draft of Dragon’s Heart.  Still, it’s good to get that down and out of your head.  When you do that you learn just what it is that you expect your character to be.  However, it is the writing that is the true test.  Seventy percent of the time I make the character a little too rude or too adventurous, for example, and I find while I’m writing that I need to tone it down a little for it to work.