Calling Reeve a "skinny little kid" when they met for his audition, director Richard Donner made Reeve promise he would be able to bulk up for the role in time for production to meet the 1978 release date.

To make sure that happened, Donner made a call to none other than Darth Vader, actor David Prowse.

The towering English actor was also a bodybuilder and weight-training instructor. He had just played the role of Vader (James Earl Jones supplied the voice) in the 1977 blockbuster A New Hope when he got the call to help Reeve.

"I get a call from Dick Donner and he says, 'Can you get down to the studio as quick as you can, we've got a Superman,'" Prowse said in a previous interview unearthed by Heat Vision.

Reeve trained with Prowse for six weeks, according to the interview.

"He was fantastic. He was a very lovely person," Prowse said of Reeve. "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."

In an interview done while he was training, Reeve said he ate four meals while on a high protein diet and he took vitamins.

"The thing is, on this part particularly, you have to start from the outside and work in," Reeve said then. "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."

He continued, "The thing is, the stronger I get — and I am still not all that strong, but I'm getting there — the stronger I get, the more it helps my mental attitude toward the part."

Reeve died in 2004.

The newest installment of the Star Wars saga, The Last Jedi, opens Thursday night.

Watch Reeve train for Superman with Prowse below: