Gregory Peck was so happy with the quality of the film, that he gave screenwriter Peter Stone a Rolls-Royce as a post-production gift after the movie came out.
During pre-production, one of the producers was set to meet with Quincy Jones, but he didn't know Jones was black. The producer saw Jones for the first time and told him, "I'll be right back," and the producer left with the intention of bringing in a non-black composer for the film. Shortly afterwards, Henry Mancini, a longtime personal friend of Jones, recommended him to the producer, thereby ensuring Jones getting hired to score the film.
Leslie Caron wrote in her book "Thank Heaven" that Universal offered her the film with Gregory Peck. But her then-boyfriend, Warren Beatty, who was controlling her life, made her turn it down. Later she regretted declining the part, as she would have loved to make a film with Peck, with whom she became friends later on.
The screenplay was written by Peter Stone as a follow-up to the hugely successful Charade (1963). Academy Award-winning actor Walter Matthau and George Kennedy were holdovers from the cast of Charade.
Director Edward Dmytryk originally wanted to cast Tippi Hedren in the part of Sheila, but Hedren was under a personal contract with Sir Alfred Hitchcock, who told Dmytryk she was not available. Diane Baker, who had played opposite Hedren in Hitchcock's Marnie (1964), took over the role.