John Gielgud was cast after director Joseph L. Mankiewicz saw him play Cassius in a stage production at Stratford-on-Avon. Mankiewicz was in Stratford to see Paul Scofield, whom he was considering casting as Marc Antony, until Marlon Brando's screen test turned out so well.
Director John Huston remarked on Brando's intense Method acting in this film as "like a hot furnace opening in a dark room."
The featurette on the DVD, "The Rise of Two Legends", is presented in anamorphic widescreen format (16:9), while the film itself is presented in its original standard "Academy format".
The film's soundtrack was actually recorded in four-track stereo, although it had not been filmed in widescreen, but the movie was eventually released in mono. If it had been released in four-track stereo, this film, and not The Robe, which was made both in Cinemascope (a screen ratio of 2.55:1) and standard "Academy ratio" (1:37:1), would have been the first motion picture released using that method of recording. "Julius Caesar" was eventually released in stereo on laserdisc and DVD.
The one scene in the play involving Cinna the Poet, in which he is mistaken for Cinna the Conspirator and killed by the angry mob, was filmed but deleted before release.