Mathieu Kassovitz developed this movie from his third short film, Assassins... (1992), which also told the story of a youth who gets a lesson in murder by a professional assassin. However, the title of the two-hour version was changed to "Assassin(s)" because of the Richard Donner film Asesinos (1995) that came out between the two.
Director Mathieu Kassovitz gives thanks to Steven Spielberg on the final credits since he was allowed to use a clip from Columbo episode Murder by the Book (1971) which was directed by Spielberg (his name appears very quick on the shot). Later on, Kassovitz appeared in Spielberg's Munich (2005).
Michel Serrault was at first reluctant to play in the film but finally accepted. During the shooting, he had so difficulties to remind his lines that when he had to do it again, they asked him to say those lines in another scene, because the first one was too hard to re shoot.