The only thing more stressful than making a film about Stephen Hawking, the most famous theoretical physicist since Albert Einstein, is watching that film while sitting next to Stephen Hawking.
"It was very, very stressful," James Marsh, director of 'The Theory of Everything,' told NBC News about watching a rough cut of the film with the famous scientist. "It was a harrowing viewing, but he was very generous and gracious about it."
The film isn't a science film. Instead, it's a love story about Stephen Hawking and his first wife, Jane, who met at Cambridge shortly before he was diagnosed with amyotrophic lateral sclerosis (ALS) at 21 years old.
It helped that Hawking made for an intriguing subject. He might be one of the most respected scientists alive today, but he had a wild streak back then.
"He liked to drink, he liked to have fun, he liked to carouse," Marsh said. "He was quite idle and, in his own book, he writes about how little work he did. He was a wastrel."
"Then the illness galvanized that brilliant mind into doing proper work, and that is the cruel irony of his condition — the illness stimulated the great work that he did, because he felt like he had so little time."
The doctors told him he had two years to live. That was in 1963.
Like many people, Marsh became enamored with Hawking through 'A Brief History of Time,' which introduced millions of readers to the concepts of black holes and the Big Bang. He also loved the documentary of the same name, directed by legendary filmmaker Errol Morris.
Still, he never thought of making a film about Hawking until he got a screenplay from Anthony McCarten, who had been impressed by Marsh's Oscar-winning documentary 'Man on Wire.'
"I wasn't sure I was the right person to do this," he said. "But when I found out that it was more a portrait of a relationship and a marriage, rather than a straight biography of Stephen and his accomplishments, I became more interested in it."
Love story or not, Eddie Redmayne, who plays Hawking, had to be convincing as a theoretical physicist, which is why the film employed one of Hawking's former students as the science advisor. That means everything — including every equation written on a blackboard or notebook — is accurate. Or, at least that is what Marsh believes.
"When the DVD is out, they can pore over it and let me know if we got something wrong," he joked.
While he made sure to get the science right, 'The Theory of Everything' isn't an educational film. The core of Hawking's ideas are often explained by Jane (Felicity Jones) with the help of potatoes, coffee and other cosmic edibles. The goal was to get across the meat of Hawking's theories without turning the film into a two-hour lecture.
"A drama isn't exactly the best place to discuss mathematics and theoretical physics," Marsh said.
"We met and he gave his blessing to the film without being particularly enthusiastic about a film based on his personal life," Marsh said.
One day, Hawking decided to stop by while the camera was rolling in Cambridge.
"He landed like spaceship on our set," he said. "It was very daunting for the actors, because he is on set, and they’re bringing his past to life. I think he enjoyed that."
It turns out Redmayne, who spent months preparing for the role, didn't need to be worried.
"He later said in an email that he felt like he was watching himself when he was watching Eddie," he said. "That is about as big of a compliment as Eddie could have received for his work."