Aaron Johnson on being John Lennon

Aaron Johnson, who plays the young Beatle in the forthcoming Nowhere Boy, talks to James Mottram about his preparation for the role – and his much-publicised romance with the film's director, Sam Taylor-Wood

No doubt Nowhere Boy will attract its fair share of Bafta nominations, if only for its magnetic central performance by the director's young beau, Aaron Johnson.

From the outside, it looked like a well-timed publicity stunt worthy of Hollywood's most cunning spin-doctors. On the closing night of this year's London Film Festival, just as the 19-year-old Aaron Johnson was soaking up acclaim for his performance as the young John Lennon in Nowhere Boy, his engagement was announced to the film's director, Sam Taylor-Wood. Together for little over six months, and with the former member of the Young British Artists some 23 years older than her leading man, needless to say it generated acres of column inches the next day.

A month on, Johnson is fidgeting and yawning his way through a promotional day for Nowhere Boy. Dressed in black jeans and tight T-shirt, he is thin, wiry and tightly coiled. On one wrist is a band with a scary-looking gold-coloured razorblade attached, while his angular, pale face is sporting a wispy beard and moustache – the sort of facial hair that teenagers tend to grow when they want to look mature. I wonder if he considers himself older than his tender years. "I'm definitely an old soul," he nods. "I don't know what my age is, if I'm honest. If you put me in a room with a bunch of people my age, I'd think they were five years younger than me."

Johnson speaks quite freely, even when it comes to talking about Taylor-Wood, who he got together with during the Liverpool-based shoot for Nowhere Boy earlier in the year. "Sam is a woman who walks into the room and lightens it up. Everyone can't help but do their best for her... she's one of the best directors I've ever worked with. She's talented." Given he's been acting for seven years – he was 12 when he landed a role in the Jackie Chan sequel Shanghai Knights – Johnson's not exactly new to the game either.

He admits his knowledge of her work was "really minimal" before he met her. "I remember being at the Tate and seeing her piece – the fruit-bowl that was rotting [2001's Still Life]." Has he learnt more since they hooked up? "Yeah, of course. When she shows me things, it gives me even more of an understanding... I see more to her personality through her past. We just have a huge connection and we talk through a load of things. But Sam pushes most of that aside, because it's her past, her art side. Like I do. We don't really talk about each other's work. We just enjoy life and what's around us now."

He and Taylor-Wood evidently take care not to tout themselves around as a celebrity couple. "We don't draw that out. We've been promoting the film, so we've been together. But we don't flash our faces around silly fashion shows or events. The only time we go to an event is to support someone. We've been out so rarely. You want your life."

Johnson seems unconcerned with the 23-year age gap or what people might think about his relationship with Taylor-Wood. "To people on the outside, who maybe judge my personal life... I'm sure it is slightly confusing. To be honest, I've never really lived by the rule book. I've had a different life to most people. I've had to be independent and take on a lot. I've had to go from family to family, and then let it go, and move onto the next project."

While he is evidently referring to the surrogate groups that actors often form when on a film, the High Wycombe-born Johnson comes from a stable middle-class upbringing. His father is a civil engineer, his mother worked in a pharmacy and his sister is now a "global technician" for Bank of America. "My mother and father were great parents. They supported me in finding what I wanted to do." Calling himself "a kid that had too much energy", he spent his pre-teens doing every after-school activity he could – karate, football, swimming and gymnastics.

For a while, he had dreams of being a pro gymnast, but after he joined a local drama club when he was 6, he changed his tune. He laments the fact that the place has changed since he was there. "Half of those [the new students] just wanna go only because they watch things like Twilight and want to be Robert Pattinson. They want fame." Does he crave a career like Pattinson's? "I feel sorry for him. He's – what? – 22. He's on 11-year-old girl's walls. He's on calendars. You know what? Fuck that." His only comparison is when he did last year's teen movie Angus, Thongs and Perfect Snogging. "I remember the producer going, 'It's going to make you a heartthrob'. I wanted to throw up. I thought, 'That sounds like a nightmare'."

Fortunately, Nowhere Boy steers him far away from being a Pattinson clone, his performance as Lennon an explosive mix of cocksure cheek, vulnerability and rage. Written by Matt Greenhalgh, who penned the Joy Division tale Control, the film is set in Lennon's teenage years at the time he truly discovered music. "It summed everything up at that point in his life – the sex, and the rock'n'roll, and the violence and the anger," says Johnson. "All of that was kept in and I think that [playing guitar] was his only way to really release it all."

While first encounters with Paul McCartney and George Harrison are all present and correct, the core of the film deals with Lennon's troubled domestic life. Nowhere Boy shows how, when he was living with his strict Aunt Mimi (Kristin Scott Thomas), he got back into contact with his more freewheeling mother, Julia (Anne-Marie Duff), who abandoned him as a child, unable to cope with the responsibility. "It's a coming-of-age story," says Johnson. "At a time when he was searching for love and trying to understand himself, he had these two strong women in his life – his aunt and his mother."

Johnson did all the preparation you might expect – two months of guitar, banjo and harmonica training as well as listening to songs from the 1950s. He also scoured the internet for footage of Lennon. With nothing of him as a teen, Johnson went to his post-Beatles phase. "I love the Beatles, but for this, I didn't need to focus on that," he says. In particular, he looked at a Rolling Stone interview where "he speaks for an hour-and-a-half, really opening up and saying what he felt, that the Beatles was a front. And how he was when he was a child, and what his aunty put him through. So I used a lot of that vulnerability from there."

Talking to Johnson, you sense that he sees Lennon as something of a kindred spirit. He cites a line from "Working Class Hero": "They hurt you at home and they hit you at school/They hate you if you're clever and they despise a fool". Then tells me, "I can empathise with Lennon. I didn't fit in at school. He said he was a genius. I wouldn't say that [about myself]. But he felt different from everybody else; he felt that they didn't understand him. He was in the wrong place." It's why Johnson didn't go to drama school or university – instead preferring to learn his craft on the job. "For me, travelling and working is the best experience you're ever going to get."

If playing Lennon has given him arthouse kudos – not least being nominated for Best Actor at the recent British Independent Film Awards – his next role looks set to propel him into the mainstream. Johnson takes the lead in Stardust director Matthew Vaughn's new film, Kick-Ass, a violent comic-book fantasy from the mind of Mark Millar, who previously penned the series that inspired the Angelina Jolie film Wanted. Johnson plays a "completely dorky nerd" who decides to become a superhero, despite lacking special powers and training. "Everyone is thinking this is a boy's film," he says, "but so far women have taken to it as much as the men."

While the film was partly shot in New York, Johnson admits he has no great love for the States and has no intention of moving to LA. He spent three months in the latter, shooting the as-yet-unreleased family drama The Greatest, alongside Pierce Brosnan and Susan Sarandon, but found the whole Hollywood lifestyle overwhelming. "I got to a point where I had to come back home. It's so business orientated. There's no soul there. That's what I personally think... it's not the place for me." Spoken like a true nowhere boy.

Nowhere Boy opens on 26 December