While director Richard Donner was initially concerned Reeve was too skinny to play Superman, he underwent a rigorous workout regimen after being cast, gaining around 30 pounds of muscle.

So while Christopher Reeve turned his role in Superman into a career-defining performance, it was his work off the screen that cemented his title as the best Superman actor of all time.

He saw that things could be changed for the better for American people,” says Ian Bonhôte, who co-directed the documentary ...