Based on the best-selling book by Bill Bryson, "A Walk in the Woods" was initially conceived by Robert Redford as a project that would reunite him onscreen with “Butch Cassidy and the Sundance Kid” co-star Paul Newman. But when Newman became ill, Redford shelved the film because he couldn't imagine doing it with anyone else – until he worked with Nick Nolte on "The Company You Keep."

The two men struck up a friendship and Redford soon realized that Nolte was born to play the role of Stephen Katz, a long-lost friend of Bryson's, with whom he had some wild times.

"I remember WWII; Bob remembers WWII," Nolte says, by way of explaining the real-life bond between them that made the movie work, especially the humor. "I've been really thinking of this now that I'm in my solid 70s. It had an extreme effect on a lot of people… And so, that was a kind of an unwritten script going on between Bob and me. I think it's this WWII experience."

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In "A Walk in the Woods," Bryson (Redford) decides that he wants to hike the Appalachian Trail. He is looking for one more adventure. Just because he's in his 70s, he doesn't feel as if his life should be over. But he can't find anyone to take the trek with until he hears from Katz, an old college buddy whom he had hasn't seen in 30 years, and with whom he has unfinished business.

"I just love the idea that a guy hit a certain point in his life and he said, 'I just have to do this thing.  I don't know why, I can't explain it, please don't ask me, but I just have to do it,'" Redford tells FOX411.

With that understanding of Bryson on which to build his character, Redford then asked the real Bryson to describe Katz, and Bryson replied, "Well, he was very smart, very daring, but he had an addictive personality." Redford admits to thinking, "Nick will be great."

To which Nolte laughing, says, "I can do that.  Not hard."

Even though Bryson never invited Katz on the journey -- Katz heard about it from a mutual friend -- he is eager to join the hike because he is avoiding legal problems back in Iowa and he has run out of luck when it comes to charming himself out of trouble. But he is overweight and too fond of the bottle to seriously tackle the 2200 miles of trail that stretch from Georgia to Maine. Still, that doesn't stop him from starting.

"Katz never had a job," says Nolte, who has growing up in Iowa in common with his character. "He didn't have a career. In the '60s, he got into the revolution and stayed revolutionary. Kind of a bum, I guess.  He didn't leave Iowa. I think that's what Katz is. He's sticking back in time. He brings that to his friend in a package that is a little confrontational, a little bit of nothingness, and a little bit of something."

Even though the actors didn't actually hike as many miles of the Appalachian Trail as it appears in the movie, nor did they camp out, but stayed in hotels, filming was challenging in more ways than one.

Nolte recalls, "We would drive maybe an hour to location, and there were camels, horses, and four-wheel drives, and Bob wanted to ride a horse. He's a horseman, he owns horses, he's a rider.  And they grabbed the reins and said, 'No, you can't have the reins, we have to lead you out there.' I got off the horse and walked up the hill, and rightly so.  But nobody would get on the camel."

"A Walk in the Woods," also starring Emma Thompson, Nick Offerman, Kristen Schaal  and Mary Steenburgen and directed by Ken Kwapis from a script by Rick Kerb and Bill Holderman, opens in theaters on Sept. 2.