Some classic episodes of television have utilized the single-set approach to storytelling, spinning out an episode almost entirely in the confines of a single room. All in the Family did it with Archie and Meathead stuck in an elevator. Homicide: Life on the Street did it with “Three Men and Adena,” the episode set mostly in an interrogation room. And tonight, This Is Us does it by confining almost all its action to a hospital waiting room. Justin Hartley and Melanie Liburd star, in what amounts to the TV equivalent of a one-act play.