I am yet to see an adapted movie that's better than the book it was adapted from. First, authors descriptions of characters paint an image in the mind of the reader. This varies from reader to reader depending on tastes, ethnicity, etc, etc. So when the movie is made, the people who do the casting don't take into account the audience expectations and in most cases they fall short of the target.
For the twilight series, I particularly wasn't impressed with the casting and script itself. Poor delivery of a tolerably good story. I wonder where they get these screenwriters. . .