Characters, Characters, Characters.
Often, movie adaptations leave out characters that played a significant role in the books. By only seeing the movie you are depriving yourself of knowing these characters who lend so much to the overall story. Some examples of movie adaptations that leave out significant characters are the Divergent Series by Veronica Roth, The Harry Potter Series by J.K. Rowling, and the Hunger Games Series by Suzanne Collins.
Now it is understandable that movies need to stray from the book a little bit in order to fit time and budget constraints, but sometimes that’s just not a good reason for major plot holes. I’ve seen film adaptations that are, in my opinion, only 20% based off of what the author actually wrote and the other 80% is totally made up. The characters may be the same, but some of the events that occur may not be in the original story at all. Plot holes sometimes make it difficult to understand the sequence of events and why things are happening. The biggest example of this that I’ve seen recently is the movie adaptation of “The Mortal Instruments: City of Bones” by Cassandra Clare. That movie just screams "plot holes!"
Depth of Relationships and Understanding What Motivates Characters.
Reading the book before seeing the movie is especially important if you value understanding the relationships of the characters. Movies can only do so much to mirror the depth of emotions between characters. In books, you have an inside perspective into the thoughts and feelings of the main character(s) and you rarely see that perspective in movies. These thoughts and emotions described in the text are very important to understanding the story as a whole. For example, in “Insurgent” by Veronica Roth, Tris’ thoughts and feelings are very important to understanding her character and why she does some of the things she does. The movies barely scratches the surface when it comes to Tris’ internal motivators.
Books ooze details on every page and paint distinctive pictures in our brains. Books thoroughly describe characters and the events that happen. So many movies leave out these details because of budget and time constraints, but sometimes it’s the details that make all the difference. Some movies just don’t seem to get character appearances correct or even location details. The simple things such as the color of a person’s hair, the layout of a person’s environment, or a specific conversation makes all the difference when reading a book compared to its movie counterpart.
If you have the time and the commitment, I suggest reading the book before seeing the movie. Nothing can beat reading a really good book for the first time. Trust me, you won't regret it.