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.
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 he well as he lent his voice to other anonymous characters. He did it all for the love of William Shakespeare and didn't accept any payment.
This was 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 Britain, was 17 at the time (the legal age of consent in Britain is 16) and did not need permission. Italy, where the film was made, has similar age laws.
It's often reported that Olivia Hussey was 15 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 Romeo and Juliet (1968) an "A" certificate, this would have made her 16 or 17 at the time of the film's release in 1968, and she legally could have viewed the picture. Even if she had been less than 16, 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 15 years old, so her youth was clearly a publicity gimmick.
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).
Although the film was originally shown with a mono soundtrack, and 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.
The last Shakespeare film (to date) to have its American television premiere on commercial network television rather than cable. Peter Brook's King Lear (1971) was never shown on commercial TV networks, and by the time Kenneth Branagh released his version of Henry V (1989), virtually every film was shown on cable TV before it went to the commercial networks.