“Other,” directed by Tony Taccone at Greenwich House Theater in Manhattan, has several themes, one being Stachel’s severe anxiety and how it has shaped his life and career. He is only 5, growing up in California, when he receives a diagnosis of obsessive-compulsive disorder, and not much older when, at the encouragement of a therapist, he names the voice in his head — the one that makes the illness’s irrational demands — Meredith, after the malevolent, would-be stepmother in the 1998 reboot of “The Parent Trap.”