After years in teen dramas and a squeaky-clean pop group, the 22-year-old has taken on the role of the notorious murderer. But changing his image is all part of the plan

Ross Lynch has just taken the strangest journey. After almost a decade as a successful teen-idol musician, touring the world with his family band R5, and as an established aristocrat in the realm of teen TV shows, he has jumped from the Disney Channel to playing serial killer Jeffrey Dahmer.

As we eat breakfast in a cafe in Sherman Oaks, Los Angeles, down the hill from the house he shares with two of his bandmate brothers, Lynch’s upbeat, clean-cut demeanour could not be further from Dahmer’s stoop-shouldered creepiness. My Friend Dahmer is based on an acclaimed 2012 graphic novel by Derf Backderf, who was at high school with Dahmer in the late 70s and was part of a coterie that, for a semester or two, adopted the future killer as a pet project. They styled themselves the Dahmer Fan Club, encouraging him in his pranks and oddball behaviour, which included “spazzing out” in school and shopping malls, unnerving bystanders by gyrating spasmodically (in imitation of the frequent withdrawal seizures of his pill-addicted mother, played here by the indispensable Anne Heche). The novel is driven by Backderf’s doubts and guilt about this period.

“The whole Dahmer Fan Club – Derf actually changed their names in the comic book – they all showed up on set, guys in their 50s now, just to kind of hang out,” says Lynch. “They were all really insightful about Dahmer. And they, too, were really shocked when it all came out later. To them, he was just a normal kid, a little out of touch, a little out of it. Derf was the one who seemed a little more cautious about him. But the rest were like: ‘Well, he was a really good tennis player …’

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“Playing him definitely affected me socially. I wasn’t as happy-go-lucky as I normally am. That walk, the stoop-shoulders, it all helped me become the guy: awkward, totally alone, cold. A lot of people who actually went to high school with Dahmer – beyond the Dahmer Fan Club guys – visited the set and complimented my walk, saying that was exactly how he did walk, to a T. A few times, Derf actually asked me to take off the glasses I wore for the movie, because he found it hard to even look at me. It was too uncomfortable for him. Derf’s whole life has been impacted by this kid that he kind of knew in high school. He’d say they were more like acquaintances, but now his whole life has Dahmer in it.”

Lynch knew next to nothing about Dahmer when he first read the screenplay – written by the film’s director, Marc Meyers – that had sat on the Black List of Hollywood’s top unproduced screenplays for a year.

“I knew very little, so I did a lot of research – his 60 Minutes interview [on the US network CBS] and other things – to get a sense of how he moved and walked and talked, a sense of how he presented himself. He was almost happy to get caught and get locked up, because it seemed like toward the end he was trying to stop what he was doing – he enlisted in the army and held down jobs and things like that.”

Lynch is the perfect ambassador for the Disney brand: polite, charismatic, fresh-faced, no swearing. I ask him if there was a strategy to obliterate that part of his image. It is a common move of former child actors, such as ex-Disney mouseketeers Ryan Gosling – lately seen stomping heads and gouging eyeballs for the director Nicolas Winding Refn – Selena Gomez and Vanessa Hudgens, who go hog-wild in Harmony Korine’s Spring Breakers. And then there is Shia LaBeouf …

Facebook Twitter Pinterest ‘It’s like Disney opens a door, you walk through it, a crowd is waiting and you’re immediately a star’ ... Ross Lynch (second left) in Disney’s Austin & Ally in 2011. Photograph: It's a Laugh Productions/Kobal/Rex/Shutterstock

“Kind of, but not really ‘obliterate’,” he says. “It was certainly very strategic, done with a sense of expanding what I’m seen to be capable of, but not necessarily to tarnish anything I’ve done in the past, which I’m proud of. It serves its purpose, gets an audience – literally, it’s like Disney opens a door, you walk through it, a crowd is waiting and you’re immediately a star. But I always had a sense that one day I’d walk up to the line and just flip the script really hard. My idea was to do a darker kind of independent movie. And this fit perfectly.”

It is not implausible for a teen idol to play Dahmer, who was a handsome boy, 6ft tall, “built like a linebacker”, according to one of the Fan Club boys. He was intelligent, too. It helps that, unlike 2002’s Dahmer, which made Jeremy Renner a hot property overnight, My Friend Dahmer takes place before Dahmer begins his psychotic pastimes. (Lynch made a point of not seeing that movie: “I heard it’s a lot more, uh … ‘Dahmer-esque’, right?” And how.)

“Any decent actor knows you can’t judge the character you’re playing,” says Lynch. “When it came to Dahmer, it wasn’t even really that hard, because he hasn’t done any of the bad things yet that he later did. Sure, he had some weird hobbies and was into some weird stuff. Just considering the family circumstances, his hellishly addicted mother and cold, distant father, and his loneliness, I didn’t really find it that hard to sympathise with Dahmer at that stage of his life. This is the becoming, all potential, nothing realised yet. This was as he was losing his innocence. The goal was just to watch him die from the inside out.”

So, how did Lynch shake off Dahmer at the end of the working day? “There were times when I would go home as Dahmer, wake up as Dahmer. But they put this blond dye in my hair that washed out. So, psychologically, watching the colour drain out of my hair and pour down the drain was like watching Dahmer disappear – that moment always felt very psychologically healthy for me.

Facebook Twitter Pinterest Ross Lynch on stage with R5 in New York in 2013. Photograph: Startraks Photo/Rex/Shutterstock

“But, to be honest, I’m almost grateful towards him: the movie just changed everything. Successful as it was, and critically acclaimed [the film was released in the US in November; it opens in UK cinemas on Friday], it put me on the map in Hollywood, which may sound surprising, since I was a Disney kid already, but that’s not being a star outside a kid’s audience.”

He is still a Disney kid, though, having recently snagged the co-star role in The Chilling Adventures of Sabrina, alongside fellow teen-TV royalty Kiernan Shipka (Sally Draper from Mad Men). He auditioned one day, re-auditioned the next and on the third day jetted off to Vancouver for 10 months of shooting – “These things happen fast!”

Meanwhile, R5, in which he is the lead singer and the lead guitarist, are heading in a direction that echoes his move to independent film: they are making a play for an indie, alternative audience beyond their Hanson-ish appeal. For now, Lynch is happy to straddle that thick, black line between winsome and evil while he ponders his next move.

“I was a Disney kid for ever and everyone liked to pigeonhole me as that – as people will tend to do – and then this comes along and completely changes my life. It’s hard to make plans, you know? But it seems to be working out.”