Every year around Christmas, Americans stop to pay homage to what is perhaps our most beloved motion picture, “It’s a Wonderful Life.” The 1946 film may flicker in black and white, but it still manages to feel fresh in affirming the human spirit as we head into each new year.

Fans of the movie might assume that making such an uplifting tale was a joy for cast and crew. In truth, this story of redeeming angels was born in the devastating wake of World War II, and it starred an actor swatting away his own demons.

The...