"I guess it goes back to 1988 when I, along with 10 million others, tried to decipher A Brief History of Time and, like most other people, I failed," laughs McCarten, referring to Stephen Hawking's bestselling science book, which tried to explain for the mass market concepts such as the Big Bang and black holes.
"In fact, it must go down as the most purchased, half-read book of all time. But what I did understand was that this was one extraordinary man, a mute genius asking from a wheelchair and with a computer voice, the biggest questions of all such as 'how did we come to be here?' and 'what is the nature of time, what is the universe?' He was like a character that leaped fully formed from the imagination of a science fiction writer and I remember thinking that one day someone would make an extraordinary feature film about this man."
Born in New Plymouth, McCarten was catapulted on to the world stage in 1987 after co-writing the international hit play Ladies' Night with Stephen Sinclair. About a group of male strippers, it became New Zealand's most commercially successful play after being translated into 12 languages and performed in more than 60 countries.
Now based mostly in London, the 53-year-old has divided his time between movies like 1999's Via Satellite and novels such as 2012's In The Absence of Heroes. In 2011, he turned that novel's prequel, his 2006 book The Death of a Superhero, into a film that starred The Lord of the Rings' Andy Serkis and Game of Thrones' Thomas Brodie-Sangster.
He read Jane Hawking's autobiography when it was published in 2004 and describes it as "a truly one-of-a-kind love story. As I turned the pages, the publicly known facts of the icon faded to reveal the powerful tale of a courageous young woman, who fell in love with a young scientist at Cambridge University only to learn that he had been given only two years to live," he recalls.
"Sure, she had the strength to support Stephen during his ordeal, she agreed to marry him, but what transpired was an extraordinary 26-year union in which they jointly explored the love of physics and the physics of love."
Overcome with "creative excitement", McCarten jumped on a train to Cambridge and knocked on Hawking's door. "I begged her to let me option the film rights to her book," he recalls. "I naively thought she would give me the movie rights that very afternoon. In fact, it took eight years."
Combining their relationship with the development of his groundbreaking scientific theories, Jane's experiences provide a crucial gateway into Stephen Hawking's otherwise seemingly impenetrable mind.
"Stephen has he didn't welcome any investigation of his private life, then Jane's book opened a door on that," says McCarten.
"The book was very candid and very unflinching about their marriage, and I just saw it as a unique and very empathetic and unusual angle with which to address a famous life. I saw the potential to make the story just as much about Jane's journey as Stephen's.
"I was aware this was an unusual approach, but I wasn't interested in doing a straight biopic. I wanted it to be an investigation of two people, a love story, which would serve as the foreground to the background story of the search for the secrets of the universe."
Penning the screenplay as well as acting as a producer, McCarten admits balancing the various elements of the Hawkings' lives was tricky and delicate.
"The film spans 30 years and there were three main storylines, all demanding equal screen time," he says. "These were an unorthodox love story, the story of the science and Stephen's long record of astonishing discoveries, and finally the horror story of ALS - amyotrophic lateral sclerosis - this brutal disease that takes and takes until you are either dead or silenced.
"Each scene therefore had to convey a great deal and this was a huge creative challenge, especially when Stephen lost the power to speak which, for a writer, is like having two arms tied behind your back."
Anthony McCarten and Stephen Hawking on the set of The Theory of Everything. Photo / Supplied
According to McCarten, integrating Hawking's groundbreaking work about the nature of the universe into the narrative proved particularly difficult.
"As to how to make the more esoteric aspects of science comprehensible, I decided to always try to have non-physicists do their best to describe the physics," says McCarten. "I thought their best efforts would be more entertaining, relatable and cinematic. I should add though that all the metaphors the lay people in the film employ were verified by our scientific adviser."
Eddie Redmayne landed the part of Stephen Hawking after a pub lunch with director James Marsh. He is an inspired choice for the lead role, undergoing an extraordinary transformation as he moves from Stephen Hawking's carefree early days as a young undergraduate to his later life when the only thing he can move is his eyelids.
"The other producers and I had agreed on a very small list of potential actors we thought were right for these parts, then James met them all and reported back," he says.
"Eddie singled himself out immediately because he is not only a very fine actor but he was also ready and prepared to do the enormous amount of preparation the role of Stephen required."
Felicity Jones, best known for playing Charles Dickens' mistress Nelly Ternan in The Invisible Woman, won the part of Jane.
"Felicity was always my first pick for Jane," continues McCarten. "When we got her and Eddie together, the chemistry was immediate and undeniable, and they are fully deserving of the plaudits they are getting."
McCarten was thrilled by Theory of Everything's Golden Globes success, which included composer Johann Johannsson winning Best Original Score. Nominated in an impressive 10 categories at next weekend's Baftas and up for five gongs at this month's Academy Awards, including Best Actor, Best Actress, Best Picture and Best Adapted Screenplay, McCarten is now trying to not get too far ahead of himself. "I think that's a bit presumptuous," he says. "But any attention that is thrown on this story of two very remarkable people I will, of course, consider a totally marvellous thing."
Widely praised by critics and a hit at the box office after grossing more than US$70 million ($95m) worldwide, McCarten is also delighted with the reception the film has received from the Hawkings.
"The tribute that has meant the most to me was seeing Stephen Hawking's tears, and then Jane's, as the house lights came up when we first showed the film to them.
"That counted, big time, and felt like the end of a journey. Stephen has recently said he finds the film a surprising, honest depiction of his marriage and that, when he was watching Eddie on screen, he forgot that he wasn't watching himself.
"At first, I don't think he was wild about the idea of the film as it was based, after all, on a book by his ex-wife. But when he saw the finished film, at the end he had tears running down his cheeks."
Stephen Hawking, Jane Hawking and Eddie Redmayne at the UK premiere of The Theory of Everything. Photo / Getty Images
In fact, Hawking was so overwhelmed with emotion he has since made his own special contribution to the film. "He began to shower the production with gifts, most notably his computer voice," continues McCarten. "So we were able to replace our approximation of that iconic voice with the real thing. Jane is now a huge supporter also, and at the London premiere she said she felt like Cinderella at the ball."
Shot on location at Cambridge University's St John's College, Cambridge is almost like a character in the story. "I grew up watching series like Brideshead Revisited, so I loved the idea of a city devoted to learning," says McCarten. "It was a dream to be given the run of St John's College, with its wedding cake-style main building, its canal ways and Venetian bridges replete with drifting punts prodded along by students. And there's the learning there that you can feel in the air."
McCarten promises added resonance for Kiwi audiences. "Isaac Newton held the chair of mathematics that Stephen Hawking would later hold, and our own Ernest Rutherford split the atom there," he says.
"We re-created the Rutherford Lab for the film, as it was dismantled years ago. When you see this scene, I hope all New Zealanders are reminded of one of our greatest sons."
• The Theory of Everything opens in cinemas on Thursday.
Next step, hacking
Anthony McCarten, father of two teenage sons, still owns a house in Wellington and returns there from his UK base for six weeks every summer.
But he hasn't crossed paths with that city's most acclaimed movie man Sir Peter Jackson for more than 20 years.
"My last dealings with him was when he cast me as a lead zombie in Braindead," he laughs. "I have watched his rise to global prominence with awe and admiration and I can't think of the precedent of any other individual having such an impact on his nation's film industry unless you go back to Louis Lumiere."
With a new novel also in the pipeline, McCarten is hard at work on a screenplay of Guardian journalist Nick Davies' Hack Attack for George Clooney to direct.
"George reached out to me as he'd read Theory and another script of mine about Winston Churchill," he says, referring to the forthcoming The Darkest Hour.
"He liked them and asked me to read Nick Davies' book, which is about the phone-hacking scandal that erupted in recent years in the UK.
"I read the book, we spoke again, after which I was given the very exciting task of adapting it for the screen. George is one week younger than me and one inch shorter, but way smarter, so it will be one of the great pleasures of my life to team up with him. "
Having spent two years working as an "inglorious hack" for the now defunct Taranaki Herald in his youth, McCarten is enjoying the opportunity to reconnect with his journalistic roots.
"It's early days so I can't say too much about it," he says.
"But if journalism becomes disconnected from the truth then we're all in deep shit, given that the entire democratic process is dependent on its sound and fair functioning."
