Watching writer-director Coralie Fargeat’s new film The Substance, I couldn’t decide whether it was supposed to be a body-horror movie or a deliciously funny All About Eve catfight about an ageing star named Elisabeth Sparkle (Demi Moore) who is outraged to discover that she is being replaced by a younger and lovelier version of herself named Sue (Margaret Qualley). What makes the film such fun is that it manages to be both.