“August Rush,” like many musicals, has been years in gestation. Berger, for example, has been on the project for close to a decade. “These things always take time,” he said, wryly, noting that he has been trying to hone his book so it zeroes in on the metaphoric core of the film, which he describes as exploring the “authentic self” in all its guises. “If you’re not in touch with that,” Berger said, “you’re not going to know how to act in this world.”