Mr. Reygadas doesn’t indicate if these rooms are real or fantasies, even if he has made it clear that Juan functions as a kind of authorial place holder and possibly a close filmmaker surrogate. This isn’t apparent from anything in the actual film, but in interviews he has explained that he shot part of “Post Tenebras Lux” in and around his house, an airy multistory modern affair set in a lush, mountainous region in south-central Mexico. Mr. Reygadas’s children, Rut and Eleazar, also play Juan’s children. (In scenes in which the children appear older, the roles are played by others.) Like Mr. Reygadas, Juan comes from money and, unlike most of the area’s inhabitants, looks to be of white European ancestry rather than of Indian descent.