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It's often reported that Olivia Hussey was fifteen years old at the movie's premiere, and thus not allowed to see it due to (her own) nudity. This is an urban myth. Her birth year is listed as 1951. The British film censor board gave this movie an "A" certificate, this would have made her sixteen or seventeen at the time of release in 1968, and she legally could have viewed the movie. Even if she had been less than sixteen, she could have attended with a guardian. It's possible that the studio could have reported this, as newspaper interviews as late as 1968 still quoted her as being fifteen years old, so her youth was clearly a publicity gimmick.
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.
Olivia Hussey revealed in her autobiography that, despite spending two years making and promoting the film, she was paid only £1500 for her work.
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.
For the scene where Juliet runs into Paris as she goes to see Friar Laurence, Olivia Hussey ran all around the set to get herself in the same agitated state she imagined Juliet would be. As Franco Zeffirelli attempted to give her direction, she told him there was no need and did the scene. After "cut" was called, she recalls a moment of silence before the director said "One Take Hussey I think we should call her from now on."