The original movie and the book have many similarities. In the movie, the events unfold in the same order as they do in the book. Also in the book and the movie, they were on an island as a bunch of boys.
The original movie and the book have many differences. In the original movie, the death of Simon and the first feast did not coincide with the book. In the book, it shows Simon hallucinating and speaking to the head of the pig on the stick, but in the movie, it did not show him speaking to the pig, just staring at it. With this, it didn’t show the author's message.