Jean Marais played both Fandor and Fantômas. In Fantômas Unleashed he's Acting for Three since he's also Professor Lefèvre (whom both Fantômas and Fandor disguise as).
Louis de Funès also played both Commissioner Juve and Fantômas disguised as Juve (with just enough makeup to have him look not too much like Juve).
California Doubling: The "Scottish" castle in Fantômas Against Scotland Yard? It's the castle of Roquetaillade, in Mazères, near Bordeaux, in France.
Fake Nationality: French actor Jean-Roger Caussimon as the Scottish Lord MacRashley in the third film.
Stunt Double: For the skydiving scene where Fandor plunges to save Juve's life at the end of Fantômas Unleashed, Jean Marais (who usually required No Stunt Double) accepted to let professional skydiver Jean-Jacques Dubourg handle it for the real freefall footage. The sequence was quite groundbreaking for its time, 14 years before a similar one in Moonraker.
What Could Have Been: A fourth movie tentatively titled Fantômas in Moscow was talked about, possibly due to the films' popularity in the Soviet Union at the time. It never got off the ground, mainly because Jean Marais had enough of being overshadowed by the booming popularity of Louis de Funès and demanded twice the salary amount he got for each previous film.
In general:
Canon Welding: Fantômas is the canonical grandfather of the supervillain Phantom Limb on The Venture Brothers, thanks to being a public domain character. (It is unknown if Helene or Fandor is his parent.)
