A book can just tell you what a character is thinking, your focus as a reader is always exactly where the writer wants it to be, because there is nothing to focus on but the words. None of that translates to movies. The broad rule for screenplays is that one page translates to one minute of runtime. You can’t directly translate a 400 page book into a screenplay. You must adapt it and make changes. Some are easy, because descriptions of how something looks like become an image. Others are difficult.
A book can just tell you what a character is thinking, your focus as a reader is always exactly where the writer wants it to be, because there is nothing to focus on but the words. None of that translates to movies. The broad rule for screenplays is that one page translates to one minute of runtime. You can’t directly translate a 400 page book into a screenplay. You must adapt it and make changes. Some are easy, because descriptions of how something looks like become an image. Others are difficult.