As the novelist Joseph Heller observed, “Just because you’re paranoid doesn’t mean they aren’t after you.” And as the three characters who barely survive Theresa Rebeck’s twisting and twisted thriller, Downstairs, demonstrate, paranoia is merely one indication that someone you know could be harboring bad intentions. Other warning signs include psychopathic tendencies, the inability to separate reality and fantasy, and sheer, anesthetizing dread. Maybe your workmates are dispensing poison, or your husband is not the man you thought you knew, or your sister has had enough. Maybe that pipe wrench would be an effective blunt instrument. Or, maybe it’s just all in your head. Rebeck and her stellar cast keep us guessing through a tense, intermission-less hour and 45 minutes, while simultaneously pondering larger questions involving inheritances of both the genetic and financial variety.