Christopher Reeve was a virtual unknown when he was cast as Superman. In fact, he wasn’t the producer’s first choice at all. Robert Redford, James Caan and Clint Eastwood all turned down the role before Reeve got the part.
When Reeve signed, he was 6’4” and 188 lbs. To prepare for the role, the producer hired David Prowse, who played the body (but not the voice) of Darth Vader in the Star Wars movies.
Reeve went through a rigid high-protein high-intensity workout regimen and he gained 33 lbs of muscle. He went from benching a personal record 40lbs to 320 lbs! He got conditioned enough to do all his own stunt work.