Theo’s “disappoint[ment]” at not catching Kathy in the act is important for several reasons. On the one hand, Theo wants to visibly see Kathy’s betrayal because he—like the titular character of Shakespeare’s
Othello—wants to affirm his own suspicions (instead of wondering if he is crazy). But perhaps less obviously, by following Kathy in this way, he is creating a link between his sense of betrayal and the betrayal Alicia recognizes in
Alcestis—and thus bringing himself closer to his favorite patient.