It is my opinion that they did choose correctly for the actors, although some of the characters (only a couple) did not fit the description the book gave. other than that they did well with the actor selection.
You know, some things in the movie doesn't match with the real stuff in the books. About the actors, yes, right choice, they are really alike the description in the books in my opinion. But the creation of the ra'zac and the urgals aren't like in the books. I see some people saying bad things about the movie, but it was nice and created the way it was possible, and it helps me very much when I'm trying to figure out some parts of the books. That's it.