Does the author have a say?

If you are adapting a book (or a series of books) for the screen - small or large - to what extent have you a duty to present your characters as they are described in the book(s)?  The latest screen portrayals of Hercule Poirot have him sporting a goatee beard.  Now those of us brought up on the David Suchet TV series - or who have read the books - know him as having a moustache, but otherwise clean-shaven.  And then there's the perennial - will the next James Bond be Idris Elba?  But he's black!  Ian Fleming describes his hero (or should that be anti-hero?) in some detail in the books, and he's clearly white.  Should it be allowed?  After all, they have given us M as a woman.  So, Poirot with a beard or a black James Bond - what do you think?  Are Agatha Christie and Ian Fleming resting peacefully in their graves, or spinning furiously?