Man and boy - The Road

For the crew that has spent this year making the film adaptation of the book – a joint production of 2929 and Bob Weinstein's Dimension Films – that meant an upending of the usual rules of making a movie on location. Bad weather was good and good weather bad. "A little fog, a little drizzle – those are the good days," special effects director Mark Forker says one morning in late April as the crew shoot on a stretch of scraggly duneland by the shore of Lake Erie.

"Today is a bad day," he adds, shaking his head and squinting.

The sky is blue, the sun so bright that crew members are smearing on sunscreen. A breeze is carrying away the fog pumping feebly from a smoke machine. Even worse, green grass is sprouting everywhere, and there are buds on the trees. Some of the crew have hand-stripped a little sapling of greenery, but the rest of the job will have to be done electronically by Forker, who is also in charge of sky replacement.

The Road began filming in late February, mostly in and around Pittsburgh, with a later stop in New Orleans and a post-production visit planned to Mount St Helens. Producer Nick Wechsler says Pennsylvania was chosen because it's one of the many states that give tax breaks and rebates to film companies and, not incidentally, because it offers a pleasing array of post-apocalyptic scenery: deserted coalfields, rundown parts of Pittsburgh, wind-swept dunes.

Production designer Chris Kennedy even discovered a burned-down amusement park in Lake Conneaut and an eight-mile stretch of abandoned freeway, complete with tunnel, ideal for filming the scene where the father and son who are the story's main characters are stalked by a cannibalistic gang travelling by truck.

The director of The Road is an Australian, John Hillcoat, best known for The Proposition, and a lot of the crew are Aussies as well. In our chats, the Mad Max movies, the Australian post-apocalyptic thrillers starring Mel Gibson, come up a lot, and not favourably. The team saw those movies, set in a world of futuristic bikers, as a sort of anti-model: a fanciful, imaginary version of the end of the world, not the grim, all-too-convincing one that McCarthy had depicted.

"What's moving and shocking about McCarthy's book is that it's so believable," Hillcoat says. "So what we want is a kind of heightened realism, as opposed to the Mad Max thing, which is all about high concept and spectacle. We're trying to avoid the clichs of apocalypse and make this more like a natural disaster." He imagines the characters less as Mad Max- style freaks outfitted in outlandish biker wear than as homeless people. They wear scavenged, ill-fitting clothing and layers of plastic bags for insulation.

The script, by Joe Penhall, is for the most part extremely faithful to McCarthy's story of a father and son travelling alone through this blighted landscape and trying to keep alive the idea of goodness and civilisation – the fire, they call it. The adaptation does enlarge and develop, in flashback, the role of the man's wife (played by Charlize Theron), who disappears quite early from the novel, choosing suicide rather than what she imagines will be starvation or worse. And of course, the script lacks McCarthy's heightened, almost biblical narrative style.

Some of that can be suggested by the look of the film, Hillcoat says, but mostly the nature of the bond between the man and the son, who in the script, as in the book, speak to each other in brief, freighted moments, will have to come out in the performances.

Viggo Mortensen, who plays the father, says the same thing. "It's a love story that's also an endurance contest," he explains, adding: "I mean that in a positive way. They're on this journey, and the father is learning from the son. So if the father-son thing doesn't work, then the movie doesn't work. The rest of it wouldn't matter. It would never be more than a pretty good movie. But with Kodi in it, it has a chance to be an extremely good movie, maybe even a great one."

Kodi is Kodi Smit-McPhee, an 11-year-old Australian who plays the son and bowled everyone over when he tested for the part, greatly reducing the anxiety filmmakers feel when casting a child. Some of the crew privately refer to him as "the alien" because of the almost freakish way that on a moment's notice he switches accents and turns himself from a child into a movie star. Days after the filming of a climactic, emotional scene, people on the set are still marvelling at Kodi's performance. One couple say they misted up just from watching the monitor and needed to sneak a tear-dabbing finger behind their sunglasses.

In the novel, the father and son have a relationship that is both tender and businesslike; they're trying to survive against great odds, after all, and there isn't much time for small talk. On and off the set Mortensen and his co-star behave in much the same way. In Erie, while Kodi's father was away for a while, Mortensen, who has a grown son of his own, moved from his suite to Kodi's room, a double, where they jumped on the beds together. During filming Mortensen, protective of Kodi, worried, for example, about yanking or dragging him too hard, but also treated him as an equal, a fellow professional who happened to have a very different way of working.

Once he emerges from his trailer, Mortensen more or less stays in character all day – bearded, gaunt, wound up and intense, going off by himself every now and then to smoke a cigarette. Kodi, on the other hand, wearing a ratty sweater, a wool cap and a pair of trousers much too big for him, wanders around and hums to himself between takes. He also engages in lengthy fencing and stick-breaking contests with Jimi Johnson, a video assistant.

For a scene in which the father, carrying the son on his shoulder, chases down a sandy road after a man who has stolen their belongings, Mortensen jogs in place to make himself seem breathless and exhausted. Kodi simply turns limp on cue, and Mortensen snatches him up like a sack.

The next scene – in which father and son catch up to the thief, and the former forces the man to take off his clothes, leaving him naked and freezing – takes forever to set up. Like neighbours at an Old West barn raising, the crew members erect a canopy over the road to cast an end-of-the-world shadow, and a while later, when the sun has moved, they have to reposition it. While waiting, Mortensen comes back and fretfully studies the monitor. Kodi, meanwhile, digs for sand beetles, showing an especially plump one to Mortensen.

"Looks like good eatin'," Mortensen says, and it isn't entirely clear whether he is joking or talking as a man who was supposed to be starving.

The thief is Michael Kenneth Williams (Omar on HBO series The Wire), one of a string of brand-name actors who turn up briefly in the film – Robert Duvall is an old, dying man, and Guy Pearce is another father wandering with his family. Williams brilliantly improvises while taking off his rags and plastic bags, pleading for his life in a way that causes the boy to take his side. When the first take is over, before a wardrobe assistant can get there, Mortensen rushes over to help Williams pick up his clothes and get dressed again.

"He's a good actor," Kodi says. Mortensen replies, "Yeah, he's good, isn't he?"

The rest of the day ticks by slowly, in a way that is a reminder that film-making may be the last vestige of 19th-century artisanal labour: hours and hours to capture what on screen would last just a few minutes. When Hillcoat calls it a wrap, a weary Mortensen heads for the makeup trailer, where he serves wine from a stash he keeps there. A while later, his face scrubbed of grime, his cheeks flushed a little, Kodi gives Mortensen a hug before heading out. Mortensen kisses him on the forehead.

"It was hard to get a rhythm out there today because of the sun," Mortensen says on the way back to his trailer. "But Kodi was unflappable, as usual. I don't even think of him as a kid. There are things he's done on this movie that I've never seen anybody do before. And there are many adult actors who never have a moment like he has every day. I can't say I've ever worked with a better partner."

He stops to snatch a hamburger, no bun, from the catering table, and after wolfing half of it, adds: "I think of Kodi as a friend. We're kind of like an old married couple. That's what our relationship is."

• The Road will be released in the UK in January next year.

McCarthy on film – a brief guide

FOR AN author so attuned to the rhythms of America – its wild landscapes, its hardscrabble history, its pervasive violence – Cormac McCarthy was ill-served by Hollywood for years.

It took the Coen Brothers' No Country For Old Men – which won four Oscars in February, including Best Supporting Actor for Javier Bardem – to show how effectively McCarthy's Faulkner-esque prose, terse dialogue and fated plots could be rendered in the right hands.

Eight years before the Coens nailed it, actor/writer/director Billy Bob Thornton laboured over an adaptation of McCarthy's sweeping 1992 cowboy novel All The Pretty Horses. But while it featured a decent cast – including Matt Damon and Penlope Cruz – Thornton's initial four-hour cut was rejected by producers, and a brokeback two-hour film crawled into cinemas in 2000 to tepid reviews. (After the runaway success of No Country, there's a rumour around that the original version may now be released.)

Next up is Australian director John Hillcoat's gritty, grizzled adaptation of McCarthy's most recent book, The Road.

Heavyweight producer Scott Rudin – one of the men who shepherded No Country to the screen – missed out on The Road, but has secured the film rights to McCarthy's 1985 novel Blood Meridian, widely regarded as one of the greatest American books of the 20th century.

Set in 1849, it centres around a runaway who falls in with a gang of scalp-hunters led by Judge Holden, a towering, charismatic man dedicated to chaos and violence. The gang roam the US-Mexico borderlands, brutally massacring Indians.

Ridley Scott is already attached to direct a script adapted from the novel by William Monaghan, who co-wrote The Departed. No casting details have been announced, but considering Scott's burgeoning working relationship with Russell Crowe, the moody Aussie might be sizing up the meaty role of Holden.

And what about McCarthy's own taste in movies? In a conversation with the Coen Brothers, the reclusive author revealed that he loved the work of Terrence Malick and Bob Rafelson. "There are a lot of good American movies, you know," he said. "I'm not that big a fan of exotic foreign films. I think Five Easy Pieces is just a really good movie."