Taranaki-born McCarten was the driving force for the film, taking years to convince Jane Hawking he could adapt her autobiography, Travelling to Infinity: My Life with Stephen, for the screen.
"I was already aware - and in awe - of Stephen Hawking from his book A Brief History of Time, but it was only when I read Jane's memoirs, in 2004, that I got a sense of the personal story behind this legendary scientist," recalls McCarten.
"Midway through reading it, I suddenly thought, 'I have to get the rights to this book'. It was such powerful, explosive material that not only dealt with an unprecedented love story but also the bigger questions in life, like the nature of time, the beginning of the universe and why we're here."
The London-based McCarten caught the first train to Cambridge to try to convince Jane Hawking - who met Stephen in the mid-1960s shortly before he was diagnosed as having amyotrophic lateral sclerosis (also known as Lou Gehrig's disease) - that she should let him be the one to bring her story to a wider audience.
Eddie Redmayne and Felicity Jones play Stephen Hawking and his first wife, Jane.
"I felt that if I didn't do something about it, I would always regret it because stories like this seldom come along. So, I raced up to Cambridge, knocked on Jane's door and tried to win her trust," he said.
"I'd never done that before - been that crazy stalker that goes and knocks on someone's door - and I've not done it since, but I felt compelled to because that's how passionately engaged I was with the material.
"It's such an incredible story that if Stephen Hawking didn't exist I don't think we'd believe it.
"If you created a character, in a wheelchair, who speaks through an automated voice and told everyone he was the most brilliant mind alive, it would instantly be dismissed as science fiction. It would be Star Trek. That's how unprecedented Stephen Hawking and this story are."
McCarten's enthusiasm took a knock when Jane Hawking rebuffed his proposal.
"I naively thought, 'I'll pop down there and try and charm the rights out of her in an afternoon'. But it took me eight years to finally get her approval, which I guess is a measure of my charm - or lack of it.
"I think it's because Jane realised that making a film about her book would lead to a greater level of global attention and she was a little afraid of that, given its sensitive, delicate nature. In the beginning, Stephen wasn't wild about the idea of a movie inspired by his ex-wife's autobiography either - what man would be? - but he read the script and eventually became a supporter, which I think is incredibly brave because it's an unflinching story and very personal."
Hawking was so moved by the script and an early version of the film that he saw, last year, in Toronto that he offered McCarten access to his unpublished autobiography and allowed him to use his computer-generated voice in the film.
"Those were such amazing gifts he bestowed upon us. We ended up running all my dialogue through his voice simulator, so I like to think that Stephen himself is in the film."
Convincing Jane Hawking was one thing, getting the film made was quite another.
"One of the obstacles was convincing people that someone in a wheelchair, who can barely move and can't talk" except through a computer - could be as exciting as a guy in a superhero costume, swinging down Madison Avenue.
"That's a big mind shift in this industry, where there's a lot of money on the table and people want to know they'll get it back at some point.
"Convincing them that an intimate, personal story like this could do that was a tough sell.
"At times, I felt like I was starting a benign religion, standing up there on a soapbox, trying to convince people that I wasn't crazy for doing this.
"But even though it's been a long, 10-year journey getting this film made, I never lost my faith in this story, its power or its importance."
It's easy to see why, because on screen, The Theory of Everything is an extraordinary, compelling tale, beautifully directed by James Marsh and featuring a harrowing performance from Eddie Redmayne, as a young Stephen Hawking, and an equally nuanced depiction of Jane Hawking by Felicity Jones.
The film is based on Jane Hawking's memoirs. Photo / AP
As one of the film's producers - another is New Zealand-born British industry veteran Tim Bevan - McCarten's name may be heard at the Golden Globes on Monday where The Theory of Everything is up for best motion picture (drama).
Then comes the Oscar nomination announcement on Friday which might make McCarten a double-nominee if the film is nominated for best picture and best adapted screenplay.
"I'm thrilled by the reaction it's received from both the press and the public.
"There's a small child in me saying, 'I told you so', because so many people doubted it. Eddie and Felicity are spectacular in it. They took my little blueprint and breathed fire into it. I'm in awe of their performances.
"But more than the critical reception it's received, it's the personal reaction to it that is really pleasing: people are profoundly moved by it.
"That had always been my hope - that it would be an emotional and inspirational experience, and it's proving to be just that. That's the biggest compliment."
He's also heartened by the reactions of the people it's about.
"When you're doing a film like this, you do have the added worry that the people it's about will see it and pass judgment but, thankfully, when that day came they were both happy with the way it turned out and felt it's an accurate, truthful depiction of their marriage," says McCarten.
"To see Stephen, with tears running down his face at the end, was incredibly humbling."
With the success of The Theory of Everything, McCarten is already entertaining plenty of offers including scripting George Clooney's new film, Hack Attack, about the British phone-hacking scandal.
"I'm in the wonderful position, today, of being in talks with three of the best directors in the world," he said.
"That's a great feeling. It's what you work so hard for, for so many years. So, for it to suddenly happen, it feels like mission accomplished."
• The movie opens in New Zealand cinemas on February 5.
Stephen Hawking
• Born: Oxford, England, 1942.
• Personal life: Married twice, three children.
• Achievements: Brilliant theoretical physicist, with groundbreaking ideas on black holes and cosmology.
• Health: Has a rare, progressive nerve disease known as Lou Gehrig's disease, which has left him paralysed. Communicates using a cheek muscle linked to a speech-generating device.
• Books include: A Brief History of Time, The Universe in a Nutshell
Anthony McCarten
• Born: New Plymouth 1961
• Novels include: Spinners (1999), The English Harem (2002), Death of a Superhero (2006), Show of Hands (2008), In The Absence of Heroes (2012), Brilliance (2012), funny girl (2014).
• Plays include: Ladies Night (1987), Via Satellite (1991)
• Films include: Show of Hands (2008), Via Satellite (1999)
