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 he reportedly lent his voice to other anonymous characters. He did it all for the love of William Shakespeare and didn't accept any payment.
Franco Zeffirelli became concerned by Olivia Hussey's full figure during filming, and ordered that no more pasta would be served on the set.
Franco Zeffirelli initially planned the film as a television production. Then Paramount became involved and increased the budget.
A rumor surrounding this film was that Olivia Hussey was really Franco Zeffirelli's daughter. Not true, of course. Franco Zeffirelli initially turned Olivia Hussey down for the role of Juliet, thinking her overweight. The actress he did choose cut her hair just before filming, ruining the effect she had on Zeffirelli. Hussey came in for another reading but by this time had developed into a beautiful teenager. She got the part.
According to Olivia Hussey, the camera used for filming, Arriflex, was very loud. As a result, the dialog had to be looped, and recorded separately later.
According to Franco Zeffirelli's autobiography, Paul McCartney was originally asked to play the part of Romeo.
Although the soundtrack on the DVD is mono, the three soundtrack albums made from this film (one with score and dialogue excerpts, one with the entire film soundtrack, and one with only the music) were all released in stereo.
During the sword fight scene when Mercutio throws a sword at Tybalt's feet, Mercutio's shadow is actually Franco Zeffirelli's shadow standing in for him because John McEnery was sick that day (according to Michael York's autobiography).
Sir Laurence Olivier dubbed the dialogue of Lord Montague.
The first major film production of this play to actually cast leading actors who were close to the ages of William Shakespeare's Romeo and Juliet. Franco Zeffirelli needed to get special permission to show teenaged Olivia Hussey topless in one scene. Leonard Whiting, who was from Great Britian, was 17 at the time (17 is the legal age of consent in Britain) and did not need permission. Italy, where the film was made, has similar age laws.