One of the problems I have with reading series of books is that the characters tend to become static after a while.  They stop growing, developing, changing, and that's when it gets boring.  How do you keep your characters continually developing for such a long series while simultaneously making sure that they still remain who they are?  I'm placing primary emphasis of this question in regards to their personalities I suppose (although physical changes effect that as well obviously).

Thanks.  I can't wait for the next book!