Often we'll hear people say things like "I liked the book better" or "Why didn't they point this and this in the movie?" or "This wasn't in the book." People constantly look film adaptations of books, plays, etc. and nitpick why they aren't exactly the same. Now most people will respond to this by saying "You can't put everything in the movie." This is a valid statement, books have the ability to be much, much longer than a movie because it is so much easier and cheaper to make. You can write just about anything you want in a book, but it is much harder to put something in a movie. However, there is a much better reason why the movie doesn't have to be exactly like the book. It is a different work of art.
The idea that two things with the same name and story are two different artistic works is a little hard to grasp at first. For example, most people, would have a hard time thinking of "Harry Potter and the Prisoner of Azkaban" and "Harry Potter and the Prisoner of Azkaban" as two different things, but one is a book and one is a movie. They are two different things. However, what if I were to tell you that during the same year, two musical theatrical adaptations of the narrative poem, "The Wild Party" were made. You wouldn't have a hard time saying that those are two different works of art. Now why is it easy to say that, but not easy to view a book and its film adaptation as different?
An obvious answer may be that it is much harder to separate a source material from an adaptation, than an adaptation from an adaptation. For an adaptation is dependent on a source material, but a adaptation is not dependent on other adaptations. However, this is reliant on the belief that the book or play that a movie is adapted from is truly the original source material. In some cases, this is blatantly inaccurate. For example, the 1969 film, "Hello Dolly!" was criticized as a poor adaptation of the musical "Hello Dolly!" However, the musical "Hello Dolly!" was in turn adapted from the play "The Matchmaker," which was in turn adapted from the play "The Merchant of Yonkers," which was in turn adapted from the farce "Einen Jux will er sich machen," which was in turn adapted from the play "A Day Well Spent." Each subsequent (no matter how reliant on a specific source material) is automatically also an adaptation of each former source material. Now with an original book being made into a movie, this isn't as easily seen. But every "original" book is really an adaptation on a basic story, a general story-line that is expanded an made into a full-fledged novel. In a way, a book, even an original one, is already an adaption.
When considering this, it makes much more sense why little things don't need to be put into movie adaptations, they were another adaptations additions to a further back source material. Therefore, making a novel and its film adaptation different artistic works that have no reason to blindly mirror each other.
