What a difference a year makes. For 19-year-old Aaron Johnson, former teen heart-throb and current screen dynamo, the life shifts of the past 12 months have been nothing less than seismic. He’s gone from being a bit-part player and tweenie poster boy — all ripped abs and smouldering stares in Angus, Thongs and Full Frontal Snogging — to starring in big-budget comic blockbusters, such as the forthcoming Nicolas Cage movie Kick-Ass, and the coveted and iconic role of John Lennon in the pre-Beatles era biopic Nowhere Boy.

He’s also been personally transformed. A restless rebellious adolescence has suddenly been quelled by a newfound appreciation of his craft, and by his romantic relationship with the director of Nowhere Boy, the artist Sam Taylor-Wood. “There is this understanding between us that is just so intimate,” he says, quietly, but flushing with pride. “It’s nice that I can sit back, and think outside the box, but with someone who understands me, and with someone I understand.”

Unsurprisingly, Johnson describes making Nowhere Boy as “one of the best times I’ve ever had in my life”. He is not, it seems, prone to snap expressions of effusion, and the words take a while to emerge.

In the flesh he is harrowingly handsome. Tanned biceps emerge from the sleeves of a green check shirt, while wisps of an early goatee cling to a face that is marble smooth and mostly conveys the epicene calm of a Greek Kouros. He even carries off a surprisingly retro haircut, Tears for Fears style, circa 1987. He is tired though, and has just stepped off a transatlantic flight from New York (“I was having a holiday, but I don’t know why so many people know about it!”). Throughout our conversation he rubs his eyes, slumps forward, toys distractedly with the crisps, apple and fizzy drink lunch in front of him, and speaks, haltingly, with the rock-star slur of someone craving a decent night’s sleep.

For now, though, it’s all about Nowhere Boy. Johnson describes reading the script while on the LA set of Matthew Vaughn’s action movie Kick-Ass (in that film he plays an American weakling called Dave Lizewski, who dreams of being a superhero). “It was the fastest thing I’d ever read. F***ing brilliant. And I could relate to it,” he says with relief, about the story, written by Matt Greenhalgh (Control), of Lennon’s early childhood. The drama, adapted from the memoir of Julia Baird (Lennon’s half-sister), tells of Lennon’s relationship with his disciplinarian Aunt Mimi (Kristin Scott Thomas) who brought him up from the age of 5, and how that relationship was fractured when Lennon’s mother Julia (Anne-Marie Duff) arrived back into his life, bringing rock’n’roll and rebellion with her.

Johnson, keen to perfect Lennon’s uniquely droll delivery (“It’s not Scouse as we know it today; but 1950s Liverpool”), launched himself on a crash course of archive material for his audition. Which, at that busy time, he confesses, was often a stretch for his own mimetic powers. “I was doing American for Kick-Ass during the day, and then during my lunch breaks I was trying to learn Liverpudlian — my head was spinning,” he says.

Once cast in the movie, he claims that he tried not to think about the sacred-cow factor, and how they — the cast and Taylor- Wood — were dealing, in new and revelatory ways, with a figure who is still internationally revered. There is one scene, for instance, where Lennon, always the wild boy, stands at the gates of a local girls’ school, unzips his trousers and proceeds to wave himself at the horrified students. “Hey! Where are you going, girls?” he taunts. “This c***’s won prizes!”

“It was a big risk for all of us,” he says. “Because not only is he an icon, but you’re also dealing with someone who’s still got family around, and you’re dealing with those family secrets. But once we started filming we just stuck together so tightly as a unit. We just had to let all those fears go, and go with out instincts. We were in a bubble.”

Johnson refers repeatedly to the making of Nowhere Boy as a bubble, and to the magical, intense and intimate things that happened within that bubble. One, of course, was the blossoming of his relationship with the 42-year-old Taylor-Wood. The pair were “outed” in Cannes, at a Nowhere Boy party, where they were photographed together for the first time. The 23-year age gap raised eyebrows. As did the fact that Taylor-Wood was newly divorced from her art dealer husband Jay Jopling, and has two daughters, aged 12 and 2.

Today, Johnson attempts, at first, to deal with the gossip head-on. “There’s a lot of wonderful relationships out there with women and younger men,” he says, toying with the ring pull of his fizzy. “Look at Vivienne Westwood [who married her boyfriend Andreas Kronthaler when he was 25 and she was 50]. Being older than me is no big deal. It’s different to how old you are in soul years.”

In the end, he decides that their relationship needs no justification. “What people say doesn’t bother us,” he says, flushing slightly. “We just stay in a bubble and stay true to each other, and strong. It just makes us stronger. It doesn’t throw us at all!” Quietly impassioned now, he begins, “And in the end, she’s the only one I can, er...” and then he suddenly stops himself because he knows, as we do, that we don’t need to know.

He nonetheless throws into relief the rest of his life, compared with this one heady year. There was an average childhood in High Wycombe, Buckinghamshire, the son of a civil engineer dad and homemaker mum, with one older sister, now a banker. He went to the Jackie Palmer stage school in High Wycombe and started acting at 6, doing McDonalds commercials, and playing son of Macduff in the Rufus Sewell production of Macbeth at the National Theatre. “I had no concept of what I was doing,” he says, of those early days. “The director told me to do something. I did it. And I enjoyed it.”

Even now, he says, he doesn’t know why he started acting at such a young age. All he knows is that he didn’t like school, and he always dreamt of escaping from High Wycombe. “I just wanted to get out, really,” he says, with a shrug. “And the best thing about acting when I was younger, was travelling. Learning more than I would’ve learnt at school just standing still!”

Along the way there were some interesting cameo roles (playing a young Edward Norton in The Illusionist), a rites-of-passage appearance in The Bill and even a misstep or two. He says, for instance, that although he was grateful for the exposure gained from playing the hunky Robbie in Angus, Thongs, it pushed him in a dangerously vapid direction. “I was this sweet charming boy who was good to look at!” he says, with an Elvis-era snarl of disgust. “It drove me mad. Doing topless shots in photoshoots. It made me realise that I don’t want to go down that road. Because it’s a short road.”

Nowhere Boy, the Closing Night Gala, will be shown on Oct 29 at Odeon Leicester Square at 7pm. The film goes on general release on Dec 25

Editor's choice

• The 100 Best Films of the Decade
• 70 facts you didn't know about Marvel
• The 50 Biggest Movies Of 2010