It's been nearly 40 years since Christopher Reeve made the world believe that a man could fly, and his portrayal of Superman remains a hallmark of the superhero movie genre. But Reeve didn't just put on the costume and become Superman. As with any role, Reeve had to prepare to play the Man of Steel, both physically and mentally. Regarding the former, when Reeve was cast as Superman, he had everything the filmmakers were looking for, with one exception: he was too skinny. To remedy that, they hired David Prowse, the man who physically portrayed Darth Vader in the original Star Wars trilogy to help Reeves bulk up for Superman: The Movie. As Prowse recalled about his time with Reeve:
He was fantastic. He was a very lovely person. We were like brothers, we got along so well together. And during the course of the period I had him, I took him from 170 pounds when we started and he was 212 [pounds] when he went into the suit.
Although James Earl Jones is well known as the voice of Darth Vader, it was David Prowse who was the one inside the Sith Lord's armor during the original Star Wars movies. Even if you've never seen Prowse outside of the Darth Vader suit, it's already clear he's an imposing man. So when in the weeks leading up to Superman: The Movie, director Richard Donner called Prowse and asked him to help train Christopher Reeve and get him into proper Superman shape. A New Hope hadn't been released by that point, but Donner knew that Prowse was a bodybuilder and weight-training instructor, so he was the right man for this particular job. As you can tell by the above comment unearthed by THR, Prowse ultimately succeeded in helping Reeve put on the weight, thus allowing him to play a physically-fitting version of Superman. When combined with Reeve's natural charm, it's no wonder why his iteration of the Kryptonian hero is still so popular.
While it's unclear precisely what type of exercises David Prowse had Christoper Reeve doing, the actor did reveal during the course of his training that he ate four means while on a high protein diet, and also took vitamins. Reeve explained:
The thing is, on this part particularly, you have to start from the outside and work in. You can do all the interior work you want to do, and it still isn't going to get you to Superman if you don't have the physical strength to go with it.
Christopher Reeve passed away in 2004, but his Superman legacy still lives on. As for David Prowse, his cinematic time as Darth Vader ended with Return of the Jedi, but he was the subject of a 2015 documentary called I Am Your Father.
You can see Henry Cavill reprise the latest silver screen version of Superman in Justice League, which is still in theaters. While Darth Vader recently appeared in Rogue One, he was portrayed in-suit by Spencer Wilding and Daniel Naprous. It's also rumored that Vader will cameo next year in Solo: A Star Wars Story.