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."