'The Road' takes Viggo Mortensen to Mount St. Helens and Astoria, Ore.

Cormac McCarthy's apocalyptic 2006 novel, "The Road," begins with a father and his young son waking in the woods to "nights dark beyond darkness and the days more gray each one than what had gone before."

In the long-delayed movie version, which opens Wednesday, the man is played by Viggo Mortensen, the boy by Kodi Smit-McPhee. The bleaker locations were played by Mount St. Helens, the Oregon coast and the woods of Pennsylvania.

"Once you're up top on [St. Helens], it's pretty bare, and you can still see the devastation of 30 years ago," said Mortensen by phone from Los Angeles. Because the production team had to wait for a break in the weather, the St. Helens scenes were among the last ones filmed.

"The last of the really grim stuff was shot in Astoria, a very beautiful area," he said, "but the day we shot it was windy, with freezing rain. The day after we finished, suddenly the sky was blue and it was really warm."

Like the book, the film never explains the cause of what appears to be the end of civilization as we know it. It's left for the filmmakers, who rely on the fallen trees at St. Helens and Spirit Lake and other visual signs of collapse, to suggest an environmental catastrophe.

"I suppose the toughest part of shooting was where we did the bulk of it, which was in the woods in Pennsylvania in the winter time, where — fortunately for [the movie] — the weather was pretty bad," he said. "We had gray or black skies all the time. It kind of wears you out if you're wet and cold outside all day."

Not that he's complaining. If you're really cold, he emphasized, you don't have to pretend, and you can concentrate on other things, like story and performances. Playing Aragorn in the "Lord of the Rings" trilogy prepared him for similar physical tests.

Mortensen sees an ecological warning in McCarthy's book and the film that Australian director John Hillcoat has made from it.

"I think it's about the things that are happening now, the climate problems we're having," he said. "But it doesn't really matter exactly how the world got there: how much is man's fault or just weather ... The characters have to face physical and emotional tests that force them to confront their individual strengths and weaknesses."

The potential for a downer story, in which, as he puts it, "everything can be taken from us," nearly scared him off when he read the script.

"It's a very faithful adaptation, and I was a little afraid of it at first," he said. "How do we keep people's interest, and really go for it? But I also knew that everything I was afraid of was going to be a learning experience."

Plus, he'd get a chance to work with Charlize Theron, Robert Duvall and Guy Pearce (who starred in Hillcoat's previous film, "The Proposition").

"Every element that was chosen, as far as the cast and the crew were concerned, was ideal," he said. Mortensen was especially impressed by Smit-McPhee, who gave a strong performance as another devastated child in the Australian father-son drama "Romulus, My Father."

"I think what he does in this movie goes beyond that," said Mortensen. "It's a lot harder role. There's a purity of emotion, a vulnerability and a presence I can't imagine from anyone else.

"I was also pleasantly surprised to find that he had a really good understanding of the book. It wasn't just luck, or a kid having a gift. He proved to be a very mature and inventive performer."

In a literal sense, "The Road" turns into a coming-of-age movie because of Smit-McPhee's performance.

"He was 10 when he started the movie, he turned 11 during the shooting, and now he's almost 13," said Mortensen, who recently turned 51. "You can almost see the changes in the movie."

John Hartl: johnhartl@yahoo.com