There's a long history of film adaptations taking HUGE licenses with the source books. Check out the novels from which Norman Bates and Forrest Gump originated. You'd hardly recognize them
The film version of "Angels and Demons" completely cut out one of the principal characters and gave his lines and actions to another character. The film version of "Absolute Power" went as far as to cut out the MAIN character and keeps the focus on the cat burglar that Clint Eastwood plays (in the book, he gets killed off partway through, which, in retrospect might have been an interesting way to shock viewers partway through).
Roald Dahl HATED the adaptation of "Charlie and the Chocolate Factory" (I agree it lacks the dark humor of the book) and therefore denied them the right to adapt the sequel. Stephen King felt the same about Kubrick's "The Shining" (Jack Torrance is supposed to be a relatively stable family man being driven to madness, but you don't cast Jack freakin' Nicholson to play someone stable).
I'd be interested to hear of any authors who LIKED the film version of their work. (The only one I can think of is William Goldman with "The Princess Bride" and that's probably because he wrote the screenplay).