The screenplay omits David Niven's part in the real operation. It was he, working for the Army's film unit as a Lieutenant Colonel, who first made contact with M.E. Clifton James. His role is taken in this movie by Major Harvey (Sir John Mills).
M.E. Clifton James denied that there was a kidnap attack on him in reality, and that this strand was added for this movie.
This is a movie based on a true story, inspired by a movie. The idea of using an actor to impersonate Montgomery in a distraction operation came from seeing Miles Mander in a supporting role in the thriller Five Graves to Cairo (1943). Intelligence leaders sought out Mander, but he was too tall for the role. M.E. Clifton James was perfect for the part.
The U.S. release title, "Hell, Heaven, or Hoboken", is an American slogan of World War I, attributed to General John J. Pershing. The city in New Jersey was the main point of departure and return for U.S. Expeditionary Forces, and the point of the phrase was that by Christmas 1917 the soldiers would either be dead or have returned in triumph. It is of course hardly relevant to a movie about a senior British Army officer of World War II. U.S. distributors were perhaps aware that Bernard L. Montgomery's personal popularity was not high Stateside. A 1984 New York Times article about him was called "The Ally We Loved To Hate".
Having lost his right-hand middle finger during World War I, M.E. Clifton James wears a prosthetic replacement.