Many of the extras were U.S. Army personnel from the 66th Military Intelligence Group who were serving in Munich, West Germany at the time of the filming. Most of the soldiers on the train at the beginning are active duty MP's who took leave to go on location in the Black Forest to make money as extras.
In real-life, George Kennedy served under General George S. Patton during World War II. He enlisted in the U.S. Army in 1943, fought in the Battle of the Bulge and was awarded two Bronze Stars. He served in the Army for 16 years rising to the rank of Captain until a back injury forced his retirement in 1959.
After the scene on the train where Col. Rogers throws Col. McCauley a briefcase, the Swiss paddle steamer Schiller can be seen on Lake Lucern. Built in 1906 with a capacity for 900 passengers, after major overhauls in 1957, 1977 and 2000, it is still in service as of 2023.
The main reason Patrick McGoohan agreed to appear in this film was because he still owed MGM one more movie on his three picture deal with the studio which began with Ice Station Zebra (1968) and The Moonshine War (1970).
The film cast includes two Oscar winners: Sophia Loren and George Kennedy; and four Oscar nominees: Max von Sydow, John Cassavetes, Robert Vaughn, and Bruce Davison.