I think it's safe to say that when you read a book and you find out there's a movie that accompanies it, there's a certain level of anticipation that comes with it. It's awesome to get to see the characters and scenarios you've been visualizing come to life, but unfortunately, these can fall flat. On the flip side, that "falling flat" feeling is one of the worst ones—especially if the book you read is one of your favorites.
The most recent example that I can personally relate is with "Wild" by Cheryl Strayed. There have been a couple of articles that I've written where I've referenced this book before. When I found out there was a movie adaption, I was pretty excited. Despite warnings from my friends, I had high hopes for it. (In my defense, there was a 90% for it on Rotten Tomatoes and a 7/10 on IMDB.)
In my humble opinion, the movie doesn't come anywhere close to being as great as the book. It felt like they heavily relied on the beginning, plucked a scene from the middle, and tied it off with the last few pages of the book. It felt very rushed. It definitely didn't deliver the same emotional impact that the book did.
But, then again, I feel like that's how it is with every book for one reason alone: You can only fit so much content into a movie that's only two or two-and-a-half hours long.
I'm not saying that the movies can't be amazing. I know this one is up for debate, but as a huge "Harry Potter" fan, I love the books AND the movies. There are, of course, some scenes (and even characters) that I wish got more time, but it's impossible to fit all of that into a movie. If they wanted to make the perfect "Harry Potter" movies, it would've taken hours for each movie (which I wouldn't really mind). Because of time restrictions, there's only so much that can get packed into a movie.
I feel like the book is, naturally, always going to be better. It's going to have that in-depth background that makes you such a huge fan of the product in the first place. The movies can do well, but nothing can ever beat the book.