At one telling moment during Jimmy Webb’s interview on “The Great Songwriters,” he comes close to giving away his composing secret. This revelation does not come easily, especially since he emphasizes the diligence and sense of mystery at the heart of his process. But when he mentions and sings “How Quickly” (from his “Twilight of the Renegades” album), Webb hints at his method, saying that a juxtaposition of opposites always works.