"It's full of poetic language that is wonderful," says the director John Hillcoat about Cormac McCarthy's best-selling novel The Road. The descriptive prose of the book combined with the post-apocalyptic setting had many believing that this was a story destined to remain on the page rather than the screen. One of those works tagged "unfilmable".

The Australian director of the much-admired The Proposition refused to listen to the naysayers, and brings to celluloid the story of a father, known only as The Man, who journeys with his son from their home in the north of the United States to the south coast. The world has been ravaged by some catastrophe, exactly what we don't know, and the road is perilous. Cannibalism is rife among fellow survivors, the cold is biting and oil is so scarce that the voyage has to be done on foot.

"By setting the story in such an extreme world, I think the book managed to highlight by contrast what really makes us human. As a father, it impacted me profoundly," said Hillcoat. "To try and stay true to that book was my number one mission."

Hillcoat knew that to veer away from the simple plot of the book and its visceral power would create animosity among its many fans. Expectations had also been ramped up by the success of the last screen adaptation of a McCarthy novel, the Coen brothers' Oscar-winning No Country for Old Men.

"We had a shortlist of writers, and actually the producers were putting forward some very big-name writers, Oscar-winning. They were thinking big and some of these guys said: 'Impossible, quit while you're ahead, it can't be done, nice talking to you,'" remembers Hillcoat. "Others, even some directors, pre-warned me: 'Don't go there, impossible', but the thing is when we had the conversation with Joe Penhall, he said what we should all realise is that we shouldn't get distracted by the legacy of McCarthy. The thing with films is you've got to be ready to fall flat on your face."

The other striking aspect of the novel that made a screen adaptation seem unlikely was the distinct lack of dialogue. The son is too young to have any conversation with his father that is beyond the superficial and the conversations with people they meet on their journey are angst-ridden discourses on survival. The only real conversations in the novel happen between The Man and his wife, which take place in flashback. The Man feels that his wife abandoned him when the going got tough.

The director argues that this was not such a big hindrance when it came to the screenplay: "The dialogue is incredibly moving. It is a love story in which they never say 'I love you', but they say it in all these other ways. We looked at it and said that is great dialogue, why can't you just edit from there to there and get it on the page and get actors to try and embody that."

A key ingredient to the success of the picture was getting the right actor to play The Man. It's hard to imagine any better choice than Viggo Mortensen. The 50-year-old actor is probably most famous for playing Aragorn in The Lord of the Rings, but it's his impenetrably austere performances in two David Cronenberg movies, A History of Violence and Eastern Promises, that showed when it comes to tough guys struggling with their emotions, who were the right mix of brawn and brain and able to do action sequences just as well as character roles, there are few better than Mortensen.

As if to reinforce the fact that this is a film that, more than most, will be judged against its source material rather than against other movies, Mortensen was wandering around after the screening I attended holding a copy of the book, which was brimming with Post-it notes. Mortensen explains: "It's a good reference. I was just looking at it. Getting my head back into it - I spent the morning for once not doing interviews, so I'm just about the movie again. I did a whole bunch of other stuff this morning - I have a book publishing company so I spent the morning dealing with other pieces of writing. Mine and other people's. I was looking at the book to get back into Cormac's world."

He feels that getting into Cormac's world was something that Hillcoat was so successful at doing that it even surpasses Peter Jackson's work on The Lord of the Rings. "It's a different medium and I haven't seen a movie where I'd read the book it was based on that was a more faithful adaptation in the best possible way - not just in spirit. Peter Jackson in spirit did a great adaptation of Tolkien, something that seemed pretty difficult and was. It was largely a question of selecting, but also he changed certain things. In this case, we didn't really change anything. The only thing you could say was an amplification - but it wasn't really in terms of screen time so much as it was by virtue of being a different medium - was that you see the mother, you hear her, you see her being played by a really good actress [Charlize Theron] who is committed to the role. I think compared with the book, you are less likely to dismiss her point of view or dismiss her as cowardly or somehow weak as a mother."

In Sundance last year, just after filming had finished on The Road, Mortensen said that he had decided to take a break from acting. The delays in the release of The Road have meant that he's remained on the cinema radar even though the actor has been as good as his word and not made a film in almost two years. In many respects, this is a performance to savour.

In an echo of the film's plot, Mortensen, in addition to working on his performance, felt responsible for ensuring that the young actor Kodi Smit-McPhee felt safe on set, especially when the production went to post-Katrina New Orleans to try to authenticate the post-disaster mood. It's when talking about Kodi that Mortensen reveals his warm-hearted side.

"I tend, like a lot of people, to make jokes when things get uncomfortable and difficult, because it's a way of dealing with it," admits Mortensen. "If he was really tired and needed some support or needed a break, I would try my hand at teasing him and usually he'd come around or give it right back - it was very difficult to get around him. He's too smart. When it comes to pranks, you can't really win in the end, he's too clever. We didn't do anything terrible, just teasing and wordplay, but more often he and I would gang up on people, particularly the cinematographer, who was a very nice man but was always very concerned, understandably, about the light."

Usually rain and clouds are an impediment when filming, but the cast and crew on The Road positively embraced it, as bad weather was vital for the look of the film's post-disaster world. "We were lucky in that almost all the time we had terrible weather," says Mortensen. "It was freakish because the last day, for example, we were already into springtime when we were shooting by the sea in the coast of Oregon. That's a wild coast and it was very, very cold that day. Kind of wintry, you know. The water was just off freezing and the temperature was 40°F (4.4°C), something like that, and there were terrible winds. But then the next day it was quite warm with blue skies, when we didn't have to shoot. We were lucky that way a lot."

One of the main concerns surrounding any adaptation is that something about the book will be drastically changed. For example when the German director Oskar Roehler adapted Michel Houellebecq's Atomised, he changed the downbeat ending for a happy one. The big question facing Hillcoat was that there is no explanation given for the earth-shattering catastrophe that created the world in which the story is set. "No, we talked about a lot of possibilities but whether it was man-made or environmental, I kind of wanted that line like in the book to be left open because once you say it's one or the other, the whole moral dilemma even changed."

Unusually, Hillcoat did not simply take the book and make the film without further consultation with the writer. But he involved McCarthy in the filmmaking process, even showing him several cuts of the movie at different stages. "He was a great ally," says the director. "When he came to the set, his son would always call him Papa. He is 73 and his son is 10 - the conversation between them is just like in the book. He never read he script, I never gave it to him and he never wanted to read it, but he would answer questions and converse. When we showed him the film he had some fantastic, very detailed comments.

"It was a great compliment and relief that he didn't feel there was anything missing from the novel except for four lines that we put back in. And he was right about them as well. It was an interesting exercise, because as soon as you physicalise something in a film, you are witnessing it as opposed to imagining it in your head when reading, and that extra physicality makes certain things unnecessary and redundant and overdone in some ways. So we steered away from certain elements that are a bit Mad Max-ish that are in the book - the slaves in chains, the boiler suits and masks, stuff like that."

Now Hillcoat must wait to see if the fans will be as happy with the result as the author.