This was the only time that David Niven ever smoked cigarettes on-screen. Niven was a life-long non-smoker.
During shooting, Gregory Peck and David Niven became close friends, bonding initially over Peck's ability to consume vast quantities of brandy, which the actors used to "stay warm" while filming in a cold studio tank, without muffing a line. Their families visited each other frequently in later years, and Peck would deliver the eulogy at Niven's funeral.
Although in the plot, Mallory is supposed to be fluent in German, Gregory Peck could not speak the language, and so his German lines are all dubbed by Robert Rietty. Peck was also dubbed when speaking Greek.
The last sequence shot was the actual setting of the bombs. With three days left to shoot, David Niven was felled by an infection from a split lip sustained shooting in the studio tank. As doctors tried to identify the infection so they could treat it effectively, the production ground to a halt for a month. Finally, Niven defied his doctors' orders and returned to the set to finish this movie before he had fully recovered. The relapse that resulted put him in the hospital for seven weeks.