Purists may cringe, but it happens: When a book is turned into a movie, some scenes and characters are shrunk or eliminated entirely, while others are expanded are introduced wholesale.
Bookish has a nifty roundup of minor characters in books whose roles got bigger in movie/TV adaptations, including Lestat in Interview with the Vampire and Irene Adler in the Sherlock Holmes series.
Interestingly, several of them are female characters in movies which would otherwise have pretty much all-male casts.
Which of your favorite movie characters started out in small roles in books?