The big picture: How Will Poulter shook off the tough tearaway tag to become Hollywood's latest loveable dork

He's only 20, but Will Poulter has already been in the middle of a snogging sandwich with Jennifer Aniston – one of the most famous women on the planet – and Emma Roberts – rising starlet and niece of Julia. OK, it was for a scene in a big new Hollywood movie. But it still represents a funny – and very public – coming of age for the young British actor. And it must have prompted more than a few high-fives from his mates…

"It's not like I managed to woo Jennifer Aniston," demurs Poulter. "It was pretty much the least romantic, least sexy thing you've ever seen in your life: people were just shouting, 'You're blocking the light' and 'Can we wipe the sweat off Will's face?'"

Still, it's quite the change of pace for a young man previously best known for his roles as the priggish Eustace in The Chronicles of Narnia: The Voyage of the Dawn Treader and as tough- nut kids in British independent movies Son of Rambow and Wild Bill.

In We're the Millers, Poulter plays a dorky, never-been-kissed 18-year-old opposite former Saturday Night Live cast member Jason Sudeikis's small-time pot-dealer, who accidentally ends up as a mule for a drugs baron. How to smuggle two tons of cannabis across the Mexican border? By playing the family guy, in chinos, driving a campervan, with an all-American wife and kids in tow – that's how.

Poulter is his neighbour Kenny, a naive teen enlisted to pose as his son, with Aniston playing a stripper-with-a-heart-of-gold fake mom, and the spunky, punky, homeless Roberts pretending to be a preppy daughter. They start off snarling, but, of course, wind up learning the true value of being there for each other. Aw shucks, etc.

The comedy is certainly broad – another of Poulter's most-discussed scenes involves a tarantula biting his balls, for which he donned a large prosthetic: "That was uncomfortable to say the least… to be wearing a swollen testicle, with your trousers round your ankles, on the side of the road: tick, tick, tick on the embarrassment checklist," he grins. But We're the Millers is frequently very funny, even if it wants both to satirise traditional family values and sentimentally promote them.

While the film doesn't quite manage to have its cake and eat it, the performances are warm and engaging, Poulter's included. Those distinctive eyebrows are permanently curved in eager-to-please anxiety, and there's a puppyish quality to him, half bounding keenness, half bashful shyness. In the US, reviews have been lukewarm, but the movie has been a surprise hit with the public, taking $26.5m on the opening weekend alone.

So convincing has Poulter been in playing vicious-looking kids with buzz-cuts in the past that it is almost a surprise that in real life, Poulter is all private-school politeness. "I was quite looking forward to not playing the brat or the bully," he says of his latest role. "There's this depiction of me, that I'm really hard – and that's not the reality at all. I am not the guy you want in a fight; I'm the guy who cries at Finding Nemo."

It is hard to think of Dean, his bruised and angry teenager in Wild Bill, weeping over animated clownfish. Dexter Fletcher's underrated 2011 directorial debut, in which Poulter gave an impressively seething performance, was set in the drug-filled estates and under-construction Olympic sites of east London, and the 20-year-old is honest about what a contrast it all was to his nice family home in Chiswick – diametrically opposite not just in terms of geography, in the west of London, but also in comfort and suburban leafiness: "Stratford – it felt a world away," he says. "But Dexter grew up in that world; it was fantastic to work with him, in that environment."

Although his performance was critically lauded, small Brit-flicks don't set gossip blogs alight. We're the Millers is a step up in the fame stakes – and Poulter's next role will likely continue the trajectory: The Maze Runner is the first in a potential Hunger Games-esque franchise based on another dystopian young-adult fiction series, and is out next February. Not that Poulter is in it for the fame: he says he chooses projects solely on the quality of the script: "Hollywood has never been a goal, necessarily – there was no 'Conquer America' game plan for me."

His experience of shooting with Aniston and Sudeikis has evidently given him plenty to think about. "Jennifer is galactically famous, but so down-to-earth and so sweet," he says. "That's really refreshing and inspiring. And it's not that people don't make it hard for her… I am so frustrated by celebrity culture," he breaks off, trying to stop himself from saying anything too controversial – but can't help himself striding forth on a topic into which he's clearly had an unnerving insight. "The fact that there aren't more restrictions on the media is disgraceful. I think it's appalling. I'm not suggesting I'd ever get to Jennifer or Jason's stage, but if I ever had more of a voice in this industry, I would campaign instantly for greater amounts of privacy and the rights of people in the public eye, because it's horrible.

"As much as I would love to do roles in big movies and am keen to establish myself in the industry," he continues, "I don't want to sacrifice things that I really enjoy, like spending time with my family and friends. I'd like to go to university, I'd like to do a bit of travelling… I don't know if I could deal with being as famous as someone like Jennifer," he concedes.

Last year Poulter began a course in drama at Bristol University, though his studies are on hold now. "I love student life, but I also loved, from a young age, being in a working environment where you are treated as an adult," he says. Nevertheless, he's going back to Bristol in October, to live in a house with – gads – seven other drama students. He has always been a bit of a junior luvvie, mind – although he visibly balks when I use the term "child actor".

Perhaps that's because he had a slightly unusual start. Poulter attended the Harrodian School – which, despite not being a performing-arts school, has turned out many a young British star (Robert Pattinson, Jack Whitehall, Tom Sturridge). Aged just 11, he starred in School of Comedy, a sketch show aimed at adults, performed by kids from his school; in London, they played above pubs none of them were old enough to drink at, then they hit the Edinburgh Fringe in 2007, becoming a sell-out success.

"I'm terrified of the idea of going on stage now; I don't know why I wasn't scared then. It was a hell of a time, and actually being on stage, live – I haven't found a feeling that matches it," he recalls. A TV series on E4 followed, and while it revealed the natural patchiness of a bunch of schoolkids, Poulter shone.

Drama was a lifeline for him even at that age, he explains; he has dyslexia and dyspraxia, and struggled at school. "It felt like it didn't matter how hard I tried, I wasn't getting anywhere. That's the most demoralising thing, as a kid. And to find something like drama, which I loved so much… it gave me a sense of purpose."

His family has always been supportive, even relocating to Australia during the shooting of the Narnia movie; Poulter, though, thinks they might just have been relieved that he found his niche. "I'm the black sheep of the family. My dad's a professor of medicine, my mum was a nurse, my little sister is going into healthcare, my older sister is a nurse, my brother's in finance – I'm the runt of the litter.

"In all honesty, as supportive as my dad is of what I do now, he was very worried about me. I was years behind [at school]; it was so frustrating to him, because he was an academic, to have a son who just couldn't do it."

These days, Poulter has that sky's-the-limit eagerness of someone for whom doors have opened early. He is hoping, even, one day to return to live comedy, despite that stagefright: "I constantly write stand-up, without having the balls to do it!" He's also written a script – a "weird little sci-fi movie" – which he's currently in talks about. It was co-authored with a pal, with whom he's also about to start shooting a super-low-budget feature film in west London; he can't give away any details, but buzzes that, "It's written by two friends of mine and it's being directed by a fantastic first-time director. It's a real passion project."

Such ventures are close to his heart and to his home, but with We're the Millers, Poulter is part of a wave of young British talent conquering America (just call it the Downton- Thrones Effect). But while he acknowledges that he has benefited from Hollywood's current transatlantic crush – he was one of a gaggle of Brits cast in The Maze Runner, including Skins' Kaya Scodelario and Love Actually's Thomas Brodie-Sangster – Poulter has his reservations about the trend: "It's funny how English actors are flavour of the month; we're as good or as bad as we've ever been. Nationality doesn't dictate talent. You should just be looking for the best actors."

Even if precocious English teenagers fall out of Tinseltown fashion, you suspect Poulter will still be hearing plenty from casting agents. At 20, the fearless kid who made audiences laugh is fast turning into a bankable star – whether he raises those eyebrows at the prospect or not.

'We're the Millers' (15) is in cinemas now