Mylène Demongeot said in her memoirs that Jean Marais was very jealous of Louis de Funès success and Marais was not so involved with the rest of the crew.
Although he as in his early 50s, Jean Marais did many of his own stunts.
For this film, Jean Marais (as Fantomas) is the dominant image on the original posters. Louis de Funes became such a hit in this film that for the second film, he had equal image size with Marais on that poster. By the third film, the image(s) of de Funes dominated the poster.
For the stunts in the motorbike, Rémy Julienne was Jean Marais's double and Louis de Funès was a dummy.
This was the first film with French super-star Louis de Funes to get a wide general release in the United States. His only other film to achieve this was released almost a decade later, "The Mad Adventures of Rabbi Jacob" (1973).