Simultaneously shot in English and French with Mel Ferrer and Christopher Lee, both of whom spoke French fluently, using their own voices on both soundtracks.
Christopher Lee was coached to perform magic tricks by Billy McComb. Even though most of them were done with camera tricks, Lee was still made a member of the Magic Circle.
Some of the action is supposed to take place in Marseille (not part of the French Riviera); but Nice (streets) and Saint-Jean Cap Ferrat (scenery) are easily recognizable.
Reportedly, Donald Pleasence's brief role was re-shot with a French actor for the French version of this film.