Los Angeles ? There’s no real secret as to how Viggo Mortensen prepared for the role of the half-starved character known as Man in “The Road.”

To lose weight, he just stopped eating as much as normal. The 51-year-old actor estimates he lost about 30 pounds that way. And, to better understand what it would be like for a man to be on a trek with his son across a barren post-apocalyptic world, the Oscar-nominated actor just thought of his own son.

“Throughout the movie I would think ‘my son did something like that once,’ after something that Kodi did,” Mortensen says. Newcomer Kodi Smit-McPhee plays his son in the film based on Cormac McCarthy’s Pulitzer Prize-winning novel.

“After awhile it didn’t matter. Thinking of my son was my way in to playing the role and then I put it away. It’s like this book. You don’t have to be a dad or a mom to relate to the predicament these people are in — this man and this child.”

Mortensen found himself having fatherly concerns about his young co-star particularly because of the harsh filming conditions. Most of the movie was shot during the winter in rural Pennsylvania to simulate a world that has gone through such a huge climatic change all life has died.

The cold weather also helped Mortensen play the role.

“If we hadn’t shot outside in the winter I don’t think it would have been as good a movie because no matter how well you fake it visually, the actors aren’t going to feel the same,” Mortensen says. “Even Kodi said it was a lot easier to be cold than pretend to be cold.

“It also affected our relationship because I felt naturally extra protective of him: not just the character but the boy himself. Here’s this skinny little kid from southern Australia who had never even seen snow.”

Mortensen’s tough physical roles in “Hildago” and the “Lord of the Rings” trilogy helped him handle that part of “The Road.” The most draining part was how the character had to be so emotionally naked.

Even the film locations tested his emotions. Some of the scenes of desolate towns were shot in areas of New Orleans not rebuilt since Hurricane Katrina. Mortensen was shocked to walk into stores and businesses where nothing had been touched, such as the office where an open briefcase with the owner’s passport still sat on a desk.

Mortensen said the real key was the script. He was attracted to “The Road” because of what it says about humanity.

“I like the idea of getting to a point where you stop making excuses, not doing the right thing. That’s what the movie is about. Man learns from what happens to them, but mainly from the boy, about forgiving oneself, about forgiving others and realizing no matter how bad things are, something good COULD happen,” Mortensen says. “And it doesn’t matter how many excuses you have for behaving in an unkind manner toward others, there’s never any excuse for not being kind. It is always better to be kind even when it seems pointless. It sounds like a very noble, ethereal, simplistic idea — but it’s true.”