To achieve the slightly skewed color scheme of the movie, the actors were made up in white face and the color palette corrected until they were flesh-toned.
The Octopus are conjoined twins joined at the leg, but the actresses who played them are not. A prosthetic third leg was designed and fitted to the actresses, but they could not walk while wearing it. This is why the Octopus sisters do not walk in any shot of their whole body.
While designing the costumes for the film, Jean-Paul Gaultier was showing co-director Jean-Pierre Jeunet the materials and fabrics he was planning to use for the different costume. Jeunet stopped him and informed Gautier that the film's budgets could not cover the cost. To which Gautier replied, "Don't worry. I'll pay for it."
Ron Perlman doesn't speak French and was the only American on set. But he learned all of his lines, and delivered them without error. In commentaries and interviews, however, he insists his French was bad.
During filming, Ron Perlman was bitten by the dog with the pulley and Judith Vittet was bitten by the mouse with the magnet.