There were many differences between the movie and the book. Some of these changes were made for the better, and some of them made it worse. These changes made it shorter and more believable. These changes will be discussed in the following paragraphs.
The boys on the island were American in the movie, not British as in the book. This change was made so it appealed to an American audience. They were in a military school before this. This made it more believable because instead of them descending from a choir group to ruthless savages, they were a military school and the drop is not as large. .
In the book the boys crash right into the island. In the movie, however; they crash into the water and use their life boat to drift to safety. This made it more believable because it is hard to believe that in the whole ocean they would land right onto an island. In the movie, Captain Benson survives but is not very mobile. Later in the movie, he is used as the "monster" that they discover. .
In the movie, Ralph injures his arm. This make it more believable because it is hard to believe that they could crash into something, and everyone walks away unharmed. .
In the movie, Simon has a very small role. He says very little, and has no small "chapel." If it wasn't for him being named right as he was being killed, I wouldn't of even know it was him. I also was disappointed with the ending, as I was with the novel. It was very weak. It does not tie up all of the loose ends.
There were also many subtle differences. In the movie it is Simon who finds the fresh water. There were no fights over the shelter, and there were fewer assemblies. In the movie they all seemed the same age. Near the end, Samneric voluntarily join Jack's group. .
The movie was also less symbolic. There were few references to the conch. Simon is also not as big of a "Christ" symbol. .
Overall, the movie was not done that well. You seem to lose a lot of the symbolism and all of the great writing that was put into the novel.