Filming didn’t get off to a great start for On the Road star Sam Riley, who plays narrator Sal Paradise in the adaptation of the Jack Kerouac classic. As the movie opens, Paradise’s father has just died, and fellow Brit Tom Sturridge, playing Carlo Marx analogue Allen Ginsberg, comes up and whispers a Hebrew dirge in his ear, an attempt at comfort.

There they were, two English guys still relatively early in their careers, excited to be kicking off the making of a movie that took decades to realize. And things went well for a few hours—until suddenly the clouds rolled in, the sky went black and the rain started pelting them like marbles. They took refuge from the thunderstorm in their trailer, wondering whether they might simply be sent home.

“We were laughing that it was Kerouac and Ginsberg pissing on us because they didn’t want two English guys playing them,” Mr. Riley told The Observer, sitting across a coffee table at the Regency Hotel.

Mr. Riley’s long, rangy figure looms, whereas Kerouac was compact, but in his sweatshirt and Levi’s he could almost pass for a dressed-down postwar college boy. “To be able to play Jack Kerouac or Sal Paradise,” Mr. Riley muses in his thick Leeds accent, “it’s mad to me.”

No discussion about American literature is complete without Kerouac’s 1957 ode to the West and its promise of freedom. And yet for all its quintessential Americaness, and its place of pride within the U.S. 20th-century literary canon, it took an international lot to finally pull off an adaptation of this supposedly unfilmable novel. Two of Mr. Riley’s highest-profile co-stars—Garrett Hedlund as Dean Moriarty and Kristen Stewart as Moriarty’s first wife Marylou—are American, but director Walter Salles is Brazilian and screenwriter José Rivera is from Puerto Rico.

Mr. Riley was born and raised in the north of England, and these days he resides in Berlin. Best known for his 2007 turn in the cult hit Control as Joy Division front man Ian Curtis, he said he occasionally draws double takes but doesn’t have the tabs following him around just yet. “I’m in a position where I can say what I say no to, but I can’t call Marty up and say, ‘Are you sick of Leo yet?’”

Before hitting the road to shoot On the Road, he’d never seen much of America outside of New York and Los Angeles. And frankly, at this point in his life, the 32-year-old would just as soon stay at home with his wife, German actress Alexandra Maria Lara. (A major part of his motive for moving to Berlin, Ms. Lara is more often recognized on the street than he is.)

“I’m quite settled now. I’ve no interest in going on a road trip,” her husband admitted. “If I want to go on holiday, I want to sit on a beach, swim, drink cocktails and read a book.”

So who does this guy think he is, playing the thinly disguised avatar of Kerouac?

“Well, don’t think I didn’t ask myself the same question,” Mr. Riley told The Observer, levering himself off the couch to grab a pack of Gauloises. He offered one, pointing out the German labels: “If you can’t understand the warning, it doesn’t affect you in the same way,” he noted.

Even if Kerouac had never written another word, the runaway popularity of On the Road would have anointed him as the prince of the Beats (though he was reluctant to wear the mantle). More important, it turned him into a kind of Saint Christopher for adolescent males, blessing their itchy feet and boldest backpacking schemes even as the country grew ever more claustrophobically suburban.

“One of the biggest parts for me was knowing that everyone would say, ‘Well, why the fuck did they hire an English guy to play Jack Kerouac?’” he admitted. Much time was invested honing his American accent, which he figured was the least he could do.

Beside the technical challenge was the sheer burden of expectation. Early in the project, he saw an interview with Johnny Depp (often cited by fans as a decent choice for the role) in which the star expressed relief that he didn’t play Sal Paradise in the film, owing to the pressure that came with it. “I remember thinking, ‘I don’t want to know, I don’t want to know about that,’” Mr. Riley said.

When Mr. Riley initially auditioned, it was 2008. Mr. Hedlund had already been cast as Dean Moriarty, but the project promptly collapsed, a victim of the financial crisis. Mr. Riley had entirely written the project off when, two years later, he got the call that the movie was on and he had the gig. “I wasn’t asked; I was just sort of told I was doing it.”

Despite their obvious differences, Mr. Riley found several ways into the character, from their common industrial upbringings to his own work as a lyricist—“not wanting to sound in any way in the same league of writing,” he was quick to disclaim.

Kerouac hadn’t exactly seen much of the country until he set out with Neal Cassady, either. “He grew up in a very sheltered environment with his mother and a father who was very dominant, and had had no experience of the great wide plains of America until he got into the road and in the car and on his own.”

“In that sense, I didn’t need to have seen it before I had to play it.”

This version of On the Road reads between the lines and reanimates the faint ghost of homosexual tension that haunts the novel. Since he’d never read the book, Mr. Riley’s first encounter with the story was Kerouac’s first draft, written in scroll form, which is more explicit. But the overt direction it takes in the movie may catch a few viewers off guard.

“I don’t think they drove around America having sex with each other, Jack and Neal, but it did happen, from what I understand,” Mr. Riley said. “In a lot of ways, they were very liberal and forward-thinking in a very conservative time and country.”

And yet the main thing most people want to ask him about is the prospect of stripping down with Ms. Stewart, the starlet who made her name in the Twilight franchise. “I’m doing interviews with Elle magazine about sex scenes with Kristen Stewart, which is all they really want to know about,” he said. Here they are adapting a counter-cultural literary classic, but the biggest point of interest is the sex scenes. “The irony isn’t lost on me,” he said.

During filming, he was also keenly aware of their 10-year age difference and their significant others. “There are nicer ways to spend an afternoon,” said the happily married star, quickly adding that he meant no offense to his co-star.

The footage in the rain was ultimately more memorable, he said. Though the scene didn’t make it into the American cut, it set the tone for the whole project: “Nothing was really going to go quite according to plan,” he said. “But there’d be lots of happy accidents that would capture the spontaneity of the prose.”