The gospel of Jack Viertel says that an opening number should set a tone; you should "acquire the vocabulary" that will help you through the evening. So, for example, "Dear Evan Hansen" opens with "Does Anybody Have a Map?" This doesn't tell us much about the characters, but it establishes a kind of frantic energy that works well for the show. Another strong example is from "A Chorus Line," a musical that is about to have its fiftieth birthday. In the canonical opening number, members of the chorus state and repeat various lines that you might expect to hear at a job interview. Because the lines are "scored," they have an unusual weight, a sense of urgency: God, I hope I get it. God, I really blew it. It could be yes, it could be no. I really need this job. Look at all the people! At all the people! The witty interlude makes a point about neurosis: The moment certain singers "make the cut," their inner monologue swings fr...