Novelists never get to see readers applaud their works—or walk out of them—but playwrights do. The dramatist’s encounter with the audience, whether disappointing or exhilarating, is a unique, indelible experience. This thought occurred to me last August, as the director and playwright Mark Rosenblatt and I hightailed it from a restaurant in London’s Soho, down the red-lanterned alleys of Chinatown and through the blinking neon hubbub of Leicester Square, to the Harold Pinter Theatre, in order to second-act the penultimate night of Rosenblatt’s West End hit, “Giant.” The play is a study of Roald Dahl, one of England’s most celebrated...