Originally, Kerouac had wanted to play Sal in the cinematic adaptation of his novel and hoped Marlon Brando would star as Moriarty and help get the film off the ground. He wasn't to see it in his lifetime - Kerouac drank himself to death in 1969. That plenty more had tried and failed to take On The Road - one earlier attempt had cast Ethan Hawke and Brad Pitt - to the big screen over the years, put all the more pressure on Riley.
"When Johnny Depp was saying how he was glad that he was too old to play Kerouac because there would be too much pressure, I thought 'well, if he is saying that, how will I fare?' Although I had a lot of anxiety and wasn't sure which way it would go, I couldn't say no, could I? Any young actor would kill for the opportunity."
Giving him the role was Brazilian director Salles, a man who already racked up some mileage in one literary road movie. He was hired by Coppola after 2004's acclaimed The Motorcycle Diaries, which depicted Che Guevera's pre-revolutionary years riding the backroads of South America.
"Sam has the intelligent sense of a writer," says Salles in praise of his leading man. "The movie's about young people trying to define what's going on, but it's also about someone trying to write a book about this process. When you are transmitting the details of the story you have to have an actor who is completely focused and who has a humanist quality and the ability to convey what's going on."
Salles also cast American actor Garret Hedlund (Tron: Legacy, Country Strong) as Dean Moriarty, the book's character inspired by Kerouac's offsider Neal Cassady, himself another Beat Generation hero who died a year before Kerouac.
Part of his audition piece for Salles was his own travelogue of a cross-country trip.
"Garrett had travelled from Minnesota to Los Angeles and he wasn't so known at the time but he'd written such a beautiful piece about the journey itself that I realised he had the sensibility for this material. His tests for the role were brilliant."
The cast was filled out by Kristen Stewart, Kirsten Dunst, Amy Adams and Viggo Mortensen, all playing figures with real-life Beat Generation compatriots - Mortensen plays Old Bull Lee, the character based on William S. Burroughs.
The cast was immersed in a pre-shoot "beat camp" to absorb what the movement was about and bond before they hit the highway.
"I am a Yorkshire man and Garrett is from Minnesota," says Riley. "We couldn't have had more different upbringings and experiences, but for whatever reason we were chosen to play these two guys at the moment when the movie finally did get made.
"That sort of glued us together because it's something that no one else understands or has gone through. Garrett and I spent six months living in each other's pockets. He was the first person I saw in the morning and the last person I saw when I went to bed. That can be hard on any friendship, never mind two egomaniac actors."
And when On the Road finally got on the road, it didn't always go to plan, care of the production's veteran vehicles.
"We broke down in Texas, we broke down in the middle of nowhere in New Mexico," Hedlund recalls with a chuckle. "We had nine different mechanics along the road."
Who: Sam Riley and Garrett Hedlund
What: On The Road
When: Opens at cinemas on Thursday
-TimeOut
