Sir Alec Guinness always felt that this was the only screen performance with which he was completely satisfied.
According to fellow actor Sir Peter Ustinov, Sir Alec Guinness was so determined to play Adolf Hitler that when he heard a rumor that Dustin Hoffman was first for the role, he went out and bought a Hitler costume. With a photographer they went to an isolated street in London to take publicity photos of him in costume. Occasional passers-by paid no attention to this, except for one policeman who wandered up to tell the actor that he was parked in a no parking area. The policeman let him off without a ticket as the policeman told him he had "no desire to spend the rest of (his) life in a concentration camp."
This film is based on the book Hitler's Last Days: An Eye-Witness Account by Gerhard Boldt (1918 - 1981). "Hauptmann Hoffmann" is a character based on Gerhard Boldt. The fourth copy of Hitler's will was a fiction. At the same time that three letters were sent out, Boldt and two others were given permission to leave. Boldt and the other friend eventually hid in a ditch in the woods then split up and returned to their families' homes.
Some British cinema chains banned this movie, as they thought it was pro-Hitler.
This was the second time Robert Rietty dubbed the voice for Adolfo Celi. This first time was in Thunderball (1965).