Film adaptations are what keeps Hollywood running, but it's a dangerous business to meddle with. Fans are touchy with their favorite books and are defensive over every detail, so it's impossible to please everyone.
Adapting to such a wildly different medium is never an easy business. It's absurd to put everything in a novel that runs for hundreds of pages into a two-hour movie. Whether it's trimming a plot, or changing something to fit the visual medium, these changes are necessary - but often determine the fate of the film.
It's not just fans who lament when filmmakers fail to adhere to the source material as closely as they'd like, but the creators. Many authors try to exert control over the movies bearing their names in the credits, for better or worse. It could be penning the script, reviewing the material, or advising the production. However, it's more common than not that their advice goes ignored, and the movies are all the poorer for it.
Some films based on books have been so atrocious that the author didn't simply refuse to endorse it, but launched a vendetta against what they saw as an insult to their work. There are cases where it's not enough just to condemn a film, but to ask moviegoers to avoid the adaptations entirely.
9. Roald Dahl - Charlie & The Chocolate Factory
Roald Dahl initially wrote the screenplay for Willy Wonka & The Chocolate Factory, but the script was so rigorously edited that he disowned the film. Dahl viewed this as hacking his work to pieces, and that those in charge were disrespecting not only the script he wrote but the original novel. In fact, most of Dahl's ideas were disregarded, like casting Spike Milligan as Willy Wonka.
Following Willy Wonka, Dahl became reluctant to allow any of his books to be put on the big screen, and he stuck to that for nearly 20 years. He loathed the adaptation of The Witches so much, he stood outside of a theater with a megaphone to deter people from going to see it.
It isn't that well known, but Charlie & The Chocolate Factory has a sequel, Charlie & The Great Glass Elevator. It has received little press compared to its predecessor because Dahl went so far as to state in his will that The Great Glass Elevator would never be made into a movie. That's why a sequel was never made to the 2005 remake, despite its financial success.