The solar eclipse that takes place during the crucifixion scene was the real thing, an event for which director Richard Fleischer delayed shooting in order to capture the ethereal nature of the phenomenon on 2/15/61.
While on location in Italy Anthony Quinn met Jolanda Addolori, a costumer on the film. They began a romance, and she gave birth to two of his sons out of wedlock. In 1966, when she was pregnant with their third child, they married.
Anthony Quinn was appearing on Broadway in "Becket" (as King Henry II) when he was approached to appear in this film. Producer Dino De Laurentiis had to buy up his Broadway contract to secure his release from the play--something Quinn was anxious for him to do, as he greatly disliked his co-star, Laurence Olivier. After Quinn had departed, Olivier continued in the play, but took over Quinn's part instead of continuing in the title role.
During the burning of Rome scene, all the city sets, built to film at eye-level angles, were torched and burned for the effect. Everything had to be shot in one night, in one take.