You’ll never be able to fit everything from a 300-page novel into a 2-hour movie. You have to cut scenes, maybe eliminate characters, and probably rework parts to make the story work for the movie.
I wish instead of seeing “based on
This is how the Bourne movies were made. I watched The Bourne Identity, liked it, and then read the book, expecting it to be similar to the movie but with more fleshed-out parts. But the book is so much more than the movie. More scenes, more locations, more challenges. The movie took the essential characters and one thread from the plot and that’s what you watch on screen. You don’t need to have read the book to understand the movie, and watching the movie does not spoil the book for you. They are separate but related, and each great stories in their own mediums.