Frank Sinatra broke one of his fingers during the fight sequence with Henry Silva. Due to ongoing 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.
In the scene where Frank Sinatra gives the all-queens deck of cards to Laurence Harvey, Sinatra is out of focus. He had trouble recreating his performance, so director John Frankenheimer left the footage as is. Audiences weren't bothered; they interpreted it as Harvey's blurred perspective.
The topic of the 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.
The scenes of the convention were filmed at the Old Madison Square Garden on 8th ave at 49th street. The last event ever held there was in Febuary 1968. It was torn down shortly after closing and today an office tower stands on the site.
Contrary to popular belief, the film was not pulled from circulation following the 1963 assassination of President John F. Kennedy. It made its American television debut on The CBS Thursday Night Movies in September 1965 (source: Broadcasting magazine), and was repeated on that network later that season. Only when the rights reverted to Frank Sinatra in 1972 did the film disappear from view, although even then turning up for third and fourth network showings on NBC in spring 1974 (source: TV Guide) and summer 1975 (source: Variety). Sinatra's neglect in keeping the film in distribution gave rise to the legend that it was suppressed because of its alleged role in Lee Harvey Oswald's assassination of the 35th president. The legend was further perpetuated when Sinatra, in alliance with MGM/UA, re-released the film to theaters in 1988. When the rumor was debunked in an article in Films in Review, another myth, one claiming that Sinatra and UA had a dispute about the profits, took its place. The myth survives to this day, but it is pure fiction.
A scene where Laurence Harvey jumps in Central Park lake was shot on February 12, the coldest day in 30 years. They had to break the foot-thick ice on the lake with a bulldozer before the scene could be shot.
Prior to the commissioning of the book as a movie, Arthur Krim, then President of United Artists and Finance Chairman of the Democratic Party, is known to have felt uneasy about its subject matter. President John F. Kennedy, as a favor to his friend Frank Sinatra, called Krim to let him know that he had no objection to a film version being made.
Famous for his use of innovative camera angles, director John Frankenheimer was widely acclaimed for a shot that is slightly out of focus. John Frankenheimer said that rather than the shot being evidence of inspiration, it was an accident and merely the best take for actor Frank Sinatra.
By his own admission Frank Sinatra's best work always came in the first take. 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 being used.
Though described by Mrs. Iselin ('Angela Landsbury') as a "a two-piece Soviet Army sniper's rifle" the weapon Raymond Shaw (Laurence Harvey) assembling is in reality a Japanese Arisaka Type 99 Type 2 Paratrooper rifle mounted with a low power rifle scope.
The trivia items below may give away important plot points.
Frank Sinatra refers to Orestes and Clytemnestra when he is talking to Laurence Harvey. Clytemnestra was the wife of Agamemnon (King of the Ancient Greek kingdom of Mycenae) who, with her lover Aegisthus, murdered him and took over the throne. Orestes, the son of Clytemnestra, later killed them both.
In Richard Condon's novel, the relationship between Mrs. Iselin and her son Raymond is more explicitly incestuous, complete with a bed scene. Director John Frankenheimer and screenwriter George Axelrod wanted to include that element, but reduced it to the less-than-motherly kiss that Mrs. Iselin plants on Raymond's lips. To appease the censors, Frankenheimer instructed Angela Lansbury to put her hand between their mouths and the camera during the kiss to obscure what she was doing a bit. By time of Jonathan Demme's The Manchurian Candidate (2004), the incestuous content between the mother and son shown on screen had been reduced even more, so that the camera cuts away before she kisses her son on the lips, only leaving the implication of that relationship between them.