Adaptations of something from a different medium are a slippery slope.
Every time there's an movie made from a book, you're going to have naysayers.
"The book was better than the movie!"
"The movie left out this part!"
"The movie made this person was SO out of character!"
And since a book relies solely on the reader's imagination to paint the scene, you're also planting an image of the characters and settings in the reader's heads that they won't be able to get rid of easily.
But that doesn't mean we shouldn't make movies out of books. It's not a matter of what you're showing, it's a matter of how you're showing it.
By nature, moviegoers don't have that long of an attention span. We have other things we need to attend to, so we can't afford to sit in a darkened room and stare at a screen for the better part of the day. Movies need to be quick to get to the point, but still have enough intrigue to keep the audience engaged.
Books are the the exact opposite. They are drawn out and long, not made to be read in a single sitting. They are meant to entertain for extended periods of time.
In 1924, Austrian-American director Erich von Stroheim attempted a literal adaptation of the novel McTeague by Frank Norris. The resulting film, Greed, was nine and a half hours long. However, the studio insisted that it be cut to a shorter length, and it was whittled down to four hours. Then, without the input of Stroheim, the movie was cut down to two hours.
The end result was a largely incoherent film, and since then, few movie directors have attempted to include every bit of a novel in the movie.
So, how can you make sure your audience doesn't lose interest and still keep much of the source material included?
TV shows are the perfect medium for adaptations of long-form works. The serialized nature of the show allows for a portion of the story to be told in one sitting each week. If we think of episodes as chapters, watching a TV show that was adapted from a book is just like reading the book itself, albeit in a purely visual format.
And if you look at the success of shows like Game of Thrones, you can see that people are much more willing to praise something that has more time to do what it set out to than they are something that needed to be concluded around a certain time.
If you can keep your audience coming back each week for a TV show, you've already done your job better than film producers.
When a film is released, it's a one-and-done deal. If there's a sequel, it's at least a year later. And with that long of a delay between installments, you're more likely to lose viewers with movies than you are with weekly TV show episodes.
And in a world where the audience matters most when it comes to success or failure?
That's a risk you can't afford to take.