Some people love to watch swans on the water. I can’t blame them, it’s a gorgeous sight. There, on the flat surface of a pond or lake, beautiful birds glide by, graceful and long-necked, pristine and white. They lift their wings more than flap. They don’t splash. There’s something perfect about the above-surface swan. Okay, but I like what makes it glide. Underneath that smooth surface, wide, waddling feet are peddling like mad to achieve what looks like effortless motion. The submerged part of the bird looks ungainly but it’s what makes the surface appearance work. That’s what I like about creative structure. Instead of the eye-capturing, realized vision, it’s the mechanism that made the imagined vision real. That mechanism is what Jack Viertel talks about in The Secret Life of the American Musical: How Broadway Shows Are Built. Like any other devotee of musical theatre, Mr. Viertel adores being swept away by a show and he’s been one of those lucky audience members for more than sixty years. He’s also been a theatrical critic, an artistic director, a producer, a dramaturg (Mr. Viertel explains a dramaturg is the “noodge” who asks questions about a developing theatrical piece that…