Once an author sells their book to be made into a movie, they have no say so over who can be in it. Some may sell the movie rights on the condition that an actor play a certain part, and some may ask for opinions from the author on casting. I found Stephenie Meyer's casting choices strange for Twilight, and none of the actors she wanted were used. An example of an author being very unhappy over a casting choice was Anne Rice. She was totally unhappy with Tom Cruise being cast as LeStat (sp) in "Interview with the Vampire," but when she saw what a good job he did, she admitted she was wrong. Not a big fan of Cruise myself, but I thought he was pretty good in it. She's retracted her opinion. I guess it depends on if they buyer of the movie wants advice from the seller of the book or not.
Probably one of my most favorite movies is "Gone with the Wind." Margaret Mitchell, the author, sold the book and sat back and let it be cast. From what I've read, she was very happy with the casting and loved the movie. I love the book too, and I don't think they could have found a much better Scarlett, Melanie, Rhett or Mammy. That has to be one of the best cast movies ever.