
I would cast relative unknowns, someone quirky enough to be able to get into the character without needing to fit into any kind of "history" they had in other films. And, I think, the story is the important part - that is why I would be making it into a movie, because I liked the book!
Oh, what book, you ask? I'm thinking one of Piers Anthony's Xanth books would make for a very entertaining movie. Plenty of multi-faceted fantastic creatures with attitude with enough of a plot to string the story along with just enough realism to give teeth to the fantasy.