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This was the first major movie production of this play to cast a leading actor and actress who were close to the ages of William Shakespeare's Romeo and Juliet. Franco Zeffirelli needed to get special permission to show Olivia Hussey, who was only fifteen, topless in one scene. Leonard Whiting, who was from Great Britain, was sixteen at the time (the legal age of consent in Britain is sixteen) and did not need permission. Italy, where this movie was made, has similar age laws.
Sir Laurence Olivier agreed to play the uncredited role of the narrator, because he was so impressed with Zeffirelli's work for the National Theatre of Great Britain, of which Olivier was director at the time. Not only was Olivier the narrator, but as Franco Zeffirelli has also confirmed, he dubbed Antonio Pierfederici's voice (due to the actor's heavy Italian accent) as well as lending his voice to other anonymous characters. He did it all for the love of William Shakespeare, and didn't accept any payment.
Olivia Hussey was put on diet pills when she was first cast as Juliet. After seeing the effects they were having on her daughter after only a few days, her mother called the studio up to inform them Olivia would not be taking the pills anymore or else they could find themselves another Juliet.
According to Olivia Hussey, the camera used for filming, Arriflex, was very loud. As a result, the dialogue had to be looped, and recorded separately later.
Olivia Hussey revealed in her autobiography that, despite spending two years making and promoting the film, she was only paid £1500 for her work.
Along with the fight between Romeo and Paris, the scene where Romeo reads the party's guest list, discovering that Rosaline would be in attendance, and the scene where Romeo visits the apothecary were also cut from the final version.