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.
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.
The trivia item below may give away important plot points.
In the DVD commentary, Ron Perlman said out of everything he'd done in his acting career, the scene where his character attacks Miette under the influence of the Octopus sisters was what he hated most.