The special lightweight rifle that The Jackal concealed in a crutch was a genuine working model. Two of them were made for this movie; one was handed over to the British authorities, the other resides in the Paris Cinematheque.
The Liberation Day scenes were filmed at a real parade, with most spectators being unaware of a movie being shot. This caused a bit of confusion: many of the crowd mistook the arrests being filmed for real ones, and attempted to assist.
Director Fred Zinnemann wanted the Jackal to be played by someone anonymous and indistinct, so he cast the relatively unknown Edward Fox. He later admitted that may have led to the film's lack success in theaters.
This movie features no soundtrack music after the first five minutes other than diegetic background music from marching bands, street musicians, and radios. Director Fred Zinnemann deliberately refused to use it on the grounds that soundtrack distracts the movement and tension generated.
When the Jackal is making an impression of a key he first rubs it against the area between his lower lip and chin, one of the more oily parts of the skin. This is because when making a clay impression the clay or the key needs to be lubricated, or the clay risks sticking to the metal and ruining the impression. The natural oils of the skin provides that lubrication here.