Eleven-year-old Hugh Edwards, who plays Piggy in the film, landed his role by writing a letter to the director which read, "Dear Sir, I am fat and wear spectacles."
Once filming began, Peter Brook largely dispensed with the script and encouraged his young cast to improvise. He shot over 60 hours of footage which was then edited down into a 90-minute film. Consequently, there is no screenplay credit.
In 1996, 35 years after the film was made, the BBC created a documentary about the making of the film called "Time Flies", which reunited the main cast and crew on the beaches of the Caribbean where it was filmed; an article written by one of the actors, Tom Gaman, mentioned that of the boys, only the one who played Ralph (James Aubrey), pursued an acting career. Others went on to have very different ones: Gaman became a freelance forester in Inverness, California; Hugh Edwards (Piggy) became an engineer for a Russian firm; Tom Chapin became a gold mine geologist in Nevada and the twins David Surtees and Simon Surtees (Samaneric) remained together, living with their families in the UK and working as a guidance counselor and political administrator respectively. (NOTE: Contrary to Mr. Gaman's article, another of the boys besides James Aubrey, Nicholas Hammond, who played Robert, had a quite extensive film AND TV career, playing, among other roles, one of the children in The Sound of Music (1965) and the title role in TV's The Amazing Spider-Man (1977).)
According to the filmmaker's commentary on the DVD version of this film (at 0:06:09), because of the loud noise from the sea and jungle on the beaches of the islands on which the movie was set, none of the dialogue could be recorded synchronously at the actual locations where the scenes were filmed. Instead, at the end of each day, the actors would be taken to a quiet location in the interior of the islands, where the dialogue for the scenes they had just filmed would be recorded from memory to be re-mixed, word by word, during the editing process. The one exception is the scene where Piggy tells some of the younger children how his hometown of Camberly got its name (which is also the only scene in the movie which is not based on a scene in the original book.)
The finished movie was given an X certificate by the British Board of Film Censors; none of the boys who'd appeared in it could see it in a theater on its first release.