Eddie Redmayne was shocked and thrilled to take home the Oscar for best actor on Sunday — beating out "Birdman" favorite Michael Keaton — for his role portraying Stephen Hawking in "The Theory of Everything."

But playing the part of the famed cosmologist came with a lot of pressure.

Hawking himself, now 73, would be judging the performance of Redmayne, who turned 33 in January.

Eddie Redmayne in "The Theory of Everything," based on the life of Stephen Hawking. Focus Features/"The Theory of Everything"

Hawking trekked to London to see the film ahead of its November 7 release. Redmayne told Variety that before watching the movie, the cosmologist warned: "I'll let you know what I think — good or otherwise." Redmayne said he answered, "Stephen, if it's otherwise, you don't need to go into details."

When the film screening was complete, Hawking was overcome with emotion (via Variety):

The real Stephen Hawking thought the film was like "watching himself." AP Photo/Ted S. Warren

After the lights came up, a nurse wiped a tear from Hawking's eye. He called the film 'broadly true,' and even celebrated with the film's director James Marsh and screenwriter Anthony McCarten at a bar where he sipped champagne from a teaspoon. 'He emailed us,' Marsh says, 'and said there were certain points when he thought he was watching himself.'

Hawking was so happy with the movie that he told filmmakers he would allow them to swap out the synthetic voice they had been forced to create and replace it with his trademarked computerized version. "We spent a lot of time and money trying to reproduce the voice, but we never got it," McCarten told Variety.

Hawking was so thrilled with the movie he gave his blessing for filmmakers to use his trademarked computer voice. "The Theory of Everything"

Redmayne was thrilled after he heard about Hawking's offer, believing it was a stamp of approval on a performance he still questioned.

"You're just hoping to get there," Redmayne said. "Yet there's this constant frustration — it's always underwhelming, because you never quite make it. But with his specific voice, it's an actor's dream. You're one step closer to the truth."

Hawking's "voice," as he explains on his website, is a computer-based communication system that has been provided by Intel since 1997. To learn more about how it works, click here.

After Redmayne won the Oscar on Sunday, Hawking posted his congratulations to Facebook:

Redmayne acknowledged Hawking and his family during his acceptance speech:

Thank you. I don't think I’m capable of articulating quite how I feel right now. I'm fully aware that I am a lucky, lucky man. This Oscar belongs to all of those people around the world battling ALS. It belongs to one exceptional family — Stephen, Jane and the Hawking children. I will be his custodian. I will be at his beck and call. I wait on him hand and foot.

"The Theory of Everything," which was made for $15 million, has raked in over $104 million at the worldwide box office since its November release.

Watch the trailer below: