In the summer of 2012 the actress Marin Ireland, Tony Award nominee, was dating Scott Shepherd, a fellow theater star. They were in London, preparing to perform as the title lovers in “Troilus and Cressida.” Offstage, though, they were arguing about their relationship. According to interviews with both, Ms. Ireland slapped Mr. Shepherd one night at their apartment; two nights later he slapped her with enough force that he knocked her to the floor.