Although this film is nowadays viewed as a cult classic, European critics at the Cannes Film Festival were so hostile to the film that director John Frankenheimer refused to leave nearby Monte Carlo, where he was shooting Grand Prix (1966), for the press conference. Rock Hudson was sent instead and was unable to answer the critical questions during the hostile session.
In order to shoot in Grand Central Station without attracting too much attention, Frankenheimer hired a male model and a Playboy bunny to make-out on the stairs while being filmed by a fake crew. This distraction allowed the real crew to shoot with a camera in a suitcase.
A scene in which Arthur Hamilton as Antiochus Wilson visits his daughter and her husband was shot, but ultimately cut from the film. The daughter was played by Evans Evans while her husband was played by Leonard Nimoy.
The depiction of Hamilton's plastic surgery includes several shots of an actual rhinoplasty operation. Director John Frankenheimer made several of these shots himself after the cameraman fainted.
John Randolph had to learn how to do many things with his left hand because Rock Hudson was left-handed.