It was especially disappointing because the original movie closely followed the novel and was much more convincing and much more enjoyable.
I would expect that when the movie rights are sold, there are different contracts. For the right amount of money, probably the producer has complete control.
I found this quote:
"Another important grant from the producer's perspective is the right to make alterations to adapt the work for a film or television production. Authors are often concerned that a producer will make changes that ruin the work or embarrass the author..... Often a compromise is reached whereby the producer agrees to consult with the author on major plot and character changes."
Margaret Atwood must understand this process well. Two of her books have recently been made into television series: "Alias Grace" and "The Handmaid's Tale".
I have heard many discussions about 'book versus movie'. Some people believe that the book is always better because you are more involved in the story as you visualize the setting and plot.
I have talked about my disappointment with "Light Between Oceans". You can read about it here. And I would agree that well-written books touch you more deeply than a movie.
However, one exception was "Angela's Ashes". The visual was very powerful! The child actors were extraordinary. I loved that movie. It had voice-overs of the actual words from the book on occasion.
The newer movies often have background noise that ruins it for me.
Sometimes the movie rights are sold before the book is finished. I believe that happened with "The Horse Whisperer".
Robert Redford bought the rights and put a different ending on it than the book.
And then there is "The Bridges of Madison County". Book versus movie? Well....
There seems to be no rule. Good books are good books...and good movies are good movies.