In the movie, there are some obvious differences with the book, however they do not affect the plot drastically. For example: In the book, Ralph has blond hair whereas in the movie he has black hair. The characters have slightly different personalities in the movie. Jack is meaner and more savage. His friendship with Ralph disappears very quickly. Besides those small details, there are some bigger differences that partially affect the story. For example: The pig head see alive and crueler than the one in the movie: when Simon finds himself in front of it, he imagines that the head is speaking to him in a mean way (the pig tells him indirectly he's going to die). Furthermore, in the movie, the pilot lives longer and dies later on when he goes into the cave and he is then referred to as the monster. In the book, the monster is know as the beast which is a parachutist that drifts to earth during a plan explosion over the island.
Other details like the relationship between two people are introduced in the movie. Simon had a very close relation with the pilot and was very trusty to him.