According to Sir Michael Caine's autobiography,"What's It All About?" (1992), Caine said of his role, the "part had originally been given to James Caan, who dropped out at the last moment. I had to finish Ilha dos Malucos (1985) on the preceding Friday night and whiz off to Berlin to start filming on the following Monday morning. It all happened so quickly, that I didn't even have time for a wardrobe fitting and wore my own clothes in the movie. Even more to the point, I didn't have time to read the script properly and, only too late, did I realize that I couldn't understand the plot, so God help the poor audience who would eventually see it."
The scene where Sir Michael Caine tells Victoria Tennant that he cannot drive a car was written especially for Caine, who does not drive.
Sir Michael Caine replaced James Caan, who walked off the production the day before filming started. Director John Frankenheimer had to film around his absent lead character, until Caine was hired as his replacement. With time on his hands, Frankenheimer shot extra footage of the assassins in the Geneva ferry scene.
In his audio commentary, director John Frankenheimer never mentions the name of the actor originally cast as the movie's lead (James Caan), but instead only refers to him as an American actor.
John Frankenheimer admitted to having had a crush on Lilli Palmer when she appeared on stage in "The Fourposter" in the 1950s. He returned to see her numerous times in the play. He confessed his youthful infatuation to her, when they worked together on this movie, and complimented her beauty, despite the actress then being in her 70s.