The Control actor features as Sal Paradise in the upcoming adaptation of Jack Kerouac's On The Road. Stewart stars as Marylou in the Walter Salles movie.
Of Stewart's reputation for being grumpy, Riley told The Guardian: "She's just 20! The tabloids can f**k off!
"She wasn't grumpy at all. She was a bit of a Rapunzel character in her hotel suite, not wanting to come to the bar to spend all evening having her photo taken."
Asked if he got stoned during his four-week stint in a 'beatnik bootcamp' for the role, Riley replied: "We were allowed to, but not encouraged. I'm a 30-year-old who used to smoke for breakfast, but the Americans smoke super-strength skunk so I stuck to my hash.
"Kerouac experts talked to us, we watched films, learned the difference between Charlie Parker and Dizzy Gillespie.
"On the Road was a hell of an experience so I'm planning to take a break. For now I'm going to be a hausfrau."