(at around 33 mins) The scene where Thorn and Roth share a meal of fresh food was not originally in the script, but was ad-libbed by Charlton Heston and Edward G. Robinson at director Richard Fleischer's request.
Knowing that he was dying, Charlton Heston brought Edward G. Robinson a variety of wines and cheeses from all over the world each day Robinson was on set for his scenes, which were all shot first. Robinson died at age 79 on January 26, 1973, four months before the film's release in May 1973.
During shooting, Edward G. Robinson was almost totally deaf. He could hear people only if they spoke directly into his ear. His dialogue scenes with other people had to be shot several times before he got the rhythm of the dialogue and was able to respond to people as if he could hear them. He could not hear director Richard Fleischer yell "cut" when a scene went wrong, so Robinson would often continue acting out the scene, unaware that shooting had stopped.
The chase scene during the climax has no dialogue because the writer, Stanley R. Greenberg, didn't want it to become an action movie. A clause in the film's contract said no dialogue could be added or edited, so they made the entire chase silent.