It's a Wonderful Life

What does this classic film suggest to us about travel between the actual and possible worlds? What does it suggest about invariance of character traits of individuals across possible worlds? Are Potter and George Bailey such invariant characters, the former always depraved, the latter always good? What was Clarence's purpose in showing George a particularly bleak alternate history of his town, one dominated by Potter's influence? How does the story reflect the wartime experiences of director Frank Capra, and star Jimmy Stewart? Is there a parallel between the life story of Jesus Christ and that of George Bailey? What lesson should each of us take away from the film?