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.
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.
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.
Janet Leigh found the role of Eugenie Rose Chaney one of the most difficult she had done because "the character was plunked down in the middle of the script, with no apparent connection to anyone, transmitting non sequiturs while sending meaningful rays through her eyes." But she was proud of her work and credited Frank Sinatra and writer, producer, and director John Frankenheimer with helping her achieve it. Modern interpretations suggest that Eugenie may also have been a double agent, but this idea was never developed in the final version.
The topic of this movie was considered politically so highly sensitive, 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 Soviet Union in 1993.