The big word-of-mouth hit around the 76th Cannes Film Festival is a French horror movie, in English and featuring American stars, about an aging, increasingly insecure actress, Demi Moore’s Elizabeth Sparkle, who has the chance to morph into a younger, “better” version of herself—with a catch, of course. That movie is The Substance, and though not everyone loves it, in a place where critics and industry professionals are loading roughly three to six films per day into their brains, a picture that people keep talking about constitutes a success.