Frank Sinatra broke the little finger of his right hand on the desk in the fight sequence with Henry Silva. Due to on-going filming commitments, he could not rest or bandage his hand properly, causing the injury to heal incorrectly. It caused him chronic discomfort for the rest of his life.
According to executive producer Howard W. Koch, the budget was $2.2 million. Of that amount, $1 million went for Frank Sinatra's salary, with another $200,000 for Laurence Harvey, leaving only $1 million for everything else.
The brainwashing sequence was filmed three times in its entirety (the garden club ladies, Corporal Allen Melvin's (James Edwards') viewpoint, and the Communist captors) against three different sets constructed so the camera could turn completely around in each. The parts were then edited together to convey the shifting perspectives.
The topic of this movie was considered politically so highly sensitive that it was censored and prohibited just before its theatrical release in many of the former "Iron Curtain" countries, such as Poland, Czechoslovakia, Hungary, Romania, Bulgaria - and even in neutral countries such as Finland and Sweden. The theatrical premiere for most of those countries was held after the collapse of the Soviet Union in 1993.
By his own admission, Frank Sinatra's best work always came in the first take. Writer, producer, and director John Frankenheimer always liked the idea of using the freshness of a first take - so nearly all of the key scenes featuring Sinatra are first takes, unless a technical problem prevented them from being used.