One of the many shocks in “The Road,” the screen adaptation of Cormac McCarthy’s post-apocalyptic novel, comes when actor Viggo Mortensen pulls off his tattered, grimy clothing to luxuriate in a waterfall.

The hunky star who played Aragorn in the “Lord of the Rings” movies now looks emaciated. His chest is sunken. His ribs seem ready to burst from beneath taut and shrunken skin.

In a recent interview, Mortensen declined to specify how much weight he lost or how he did it.

“Just doing my job,” he said. “It would have been so wrong to show up looking like I’d just had a nice lunch. But the physical stuff was only a means to an end. The cold, the wet, the hunger ... all that helped us to get to another place. An emotional place.”

Fans of McCarthy’s Pulitzer-winning novel have anticipated the arrival of John Hillcoat’s movie version (it opens Wednesday) with both hope and foreboding. After all, Hollywood has a long history of wussing out when the going gets tough.

And this is a tough book.

It’s about a nameless father and son wandering through an unending winter. Some unspecified disaster — nuclear war, meteorite strike, climate change — has left the sky perpetually overcast. All plants have died and with them wildlife. The man and boy will probably starve, providing they aren’t first done in by roving cannibal gangs.

The novel is almost too painful to absorb. Making it bearable is McCarthy’s sparse, practically poetic prose and the story’s emotional core of paternal love and hope against all odds.

Could Hollywood possibly take on material this grim without turning “The Road” into “The Road Warrior”? Terrified of the material, would the moviemakers glamorize and romanticize? Throw in a few big action scenes? A sneering villain? Maybe a love interest?

Mortensen said he’d read most of McCarthy’s books but hadn’t gotten around to “The Road” when he was given Joe Penhall’s screenplay.

“I read the script, was knocked out, and ran right out to pick up the novel. I wanted to know how faithful the script was to the book. It was very faithful, and it only became more faithful as we went into production and began paring things down.

“When I saw the finished picture I found it was the most faithful screen adaptation of a novel I’ve ever seen — and that includes ’Lord of the Rings,”’ he said. “For the reader of this book, there’s no shortcut. There couldn’t be one for us, either. The emotional journey has to be real.”

Mortensen said his pairing with young Kodi Smit-McPhee, who was only 10 when he was cast as the Boy, resulted in the most meaningful acting collaboration of his career.

A father in real life, Mortensen found himself developing a protective paternalism toward his young co-star, with whom he shares practically every scene in the film.

“It’s odd — the Man is desperately trying to keep the Boy alive, and the Boy’s presence is keeping the Man alive. The same thing happened on the set.

“What I hadn’t anticipated was how Kodi’s performance allowed me to be strong and vulnerable. No matter what I put out there, if the Boy isn’t great, I can only go so far. But once I recognized what we had in Kodi, I realized I could take any risk and he’d always be there to catch me. It was like he was my protector.”

Mortensen was particularly impressed by young Smit-McPhee’s ability to portray a child born after the great disaster — one who had never experienced TV, radio, recorded music or most consumer goods.

“It’s not that easy for a kid living in today’s world to grab a can of Coke without giving away that he knows what it is, or to show what it’s like to take your first sip of a soda.

“Adult characters are all the things they’ve encountered over time. But kids haven’t accumulated all the life experience, all the regrets. They tend to be more in the moment, more willing to play, to be joyful. Kodi absolutely had all that. And because his character is totally in the moment, the father is forced to be there, too.”

For two weeks before filming, Hillcoat (he directed “The Proposition,” an uber-violent Australian “Western”) sequestered his two stars and writer Penhall in a Pittsburgh hotel. Mortensen explained that “The Road” was modestly budgeted (about $20 million) and that once the camera began rolling, there was no time for experimentation or getting in the mood.

“We knew we’d be shooting outdoors most of the time and dependent on the weather and limited hours of winter light in which to film. Plus if the sun peeked out we had to wait for it to go under. Nobody in this film could cast a shadow. So you had to be ready when the sky cooperated.

“It’s the only movie set I’ve been on where we were all praying for bad weather.”

The rehearsal period gave the actors an opportunity to explore the characters, including their previous lives. Though the film has several short flashbacks of the Man and his wife (Charlize Theron) in the aftermath of the disaster, neither it nor the book gives any substantial background on the characters.

“Kodi and I gave ourselves names, though they’re never spoken in front of the camera,” Mortensen said. “For the Man I invented a life between his birth and Page 1 of the script. His name, the town he was raised in, his career. Charlize and Kodi” — she’s South African, he’s Australian — “have similar American accents that are different from mine. So I invented a backstory that I married her and then we moved back to her hometown.

“Usually when you do research for a film you’re bringing it to the set, accessing it. But this time all the inventions I came up with had to be forgotten as soon as they said ’action.’ The audience doesn’t know any of this stuff, but it all helped me understand where the Man came from.”

“The Road” is such a dark tale that its commercial possibilities seem limited. Yet Mortensen said that test screenings he attended indicated that once seen, the movie gets under people’s skins.

“We had Q&A; sessions after the movie, and I’ve never had so many audience members confess so many secrets to me. I think it’s because the film forces you on this emotional journey and makes you ask big questions.

“What does it mean to be one of the good guys? What’s the point in being alive, what’s the point of surviving? This story makes us come face to face with our own strengths and weaknesses. It tests your ability to be loving under the most dire circumstances.

“It’s a simple thing to say that it all boils down to love. A cliche, really. And yet at the end of this story we’re moved and uplifted because it confirms something we knew all along: Kindness matters”