Andrzej Chyra as Hippolytus and Isabelle Huppert as Phaedra in the Sarah Kane section of Phaedra(s) at BAM (photo by Pascal Victor/ArtComArt, courtesy Odeon-Theatre de l’Europe)

Though the ability to play multiple roles is essential to the art of acting, there’s something uncanny about seeing the switch happen before us. Even when we know there’s some presentational trickery involved, as when separate performances by Tatiana Maslany are composited into a single scene on the BBC America drama...