Many books have been turned into movies. A few movies have been turned into books. Find one you like, read the book, then watch the movie.
For instance, I read the book Jurassic Park, then watched the movie. Much of the movie was identical to the book, but there were some differences. The same is true of most movie-book productions.
When done, compare the movie to the book. Did the film-maker make the film the way you imagined it while you were reading? Were the characters the same? Did you like some of the characters better in the book or in the movie? What parts of the book were left out? Was anything new included in the movie? Did the film-maker change anything from the way it was printed in the book? If there are differences between the book and the movie, why do you think the film-maker made these changes?