When Robert Redford originally purchased the rights to Bill Bryson's 1998 memoir A Walk in the Woods, he intended it as a reunion film for himself and his lifelong friend Paul Newman. The duo's most iconic pairings were in Butch Cassidy and the Sundance Kid (1969) and The Sting (1973). But Newman's illness made the role impossible, and when he died in 2008 the film almost died with him.

Robert Redford and Nick Nolte in 'A Walk In The Woods'Robert Redford and Nick Nolte take one hell of a hike in 'A Walk In The Woods'

Then Redford met Nick Nolte in 2012 and cast him in his political thriller The Company You Keep. During filming, Redford realised that he'd be perfect as Bryson's messy, out-of-shape hiking pal Katz in A Walk in the Woods. "I liked him as an actor," Redford says. "You could see that he had an undisciplined side in life."

More: Watch the trailer for Nick Nolte's other film from this year 'Return To Sender'

"Our backgrounds were very similar before I got my act together," Redford says. "I got my act together somewhere along the line, but earlier in my life I was a mess, out for adventure and risk. I pulled it together, but I could identify with that part of Nick."

Watch the trailer for 'A Walk In The Woods' here:

Shooting the movie wasn't exactly easy. It's about two retirement-age men taking on the 2,160-mile Appalachian Trail from Georgia to Maine. And although Redford and Nolte didn't actually hike the whole trail, Redford says, "It felt like we did because we would hike sections going uphill and, because it's film, you do one take and then you go back and do it again, then you do it again. After six, seven, eight takes, you're really pooped!"

Redford just turned 79 and says he has no intention of retiring any time soon. He likens his career to taking the hike: "Don't look back, and don't stop!"