Life after Harry Potter: Look what’s happened to Neville Longbottom – it’s magic!

There’s still no sign of Matthew Lewis, more than an hour after his train was due into London’s King’s Cross. Either he’s metamorphosed yet again, into an even better-looking version of his younger self, and I’ve missed him, or he’s paid a trip to Platform 9 ¾ to rekindle some old memories.

Lewis, now 24, found fame playing the cupcake-faced Neville Longbottom in all eight Harry Potter films, not that you’d know that by looking at him. Blog after blog hails him as the proverbial swan, his “before and after photos” standing testimony to the power of hormones. Or as the US satirist Stephen Colbert put it: “This is a real message of hope.”

Even Lewis, once he finally turns up, is at it, whipping out his phone to show me “the freakiest picture” someone sent him the other day. “It’s a superimposed picture of me at the final premiere and a film four premiere. It’s terrifying. I had to save it because it made me so uncomfortable,” he adds, in his thick Leeds accent.

His delay was the fault of East Coast trains, wiping out the point of his 5.45am alarm. Despite claiming he is tired, he throws himself into our interview, all smiles and charm, sipping only the St Pancras Hotel’s green tea by way of revival aid. He insists the early start was helpful because in two days’ time he needs to be in Newcastle by first thing for the Great North Run, which he is doing to raise money for his beloved Leeds Rhinos and its local charity foundation (donations welcomed).

For the record, he tries to avoid Platform 9 ¾ but because King’s Cross has located the fictional landmark near the left-luggage office he can’t always do so. “It’s the most awkward thing, walking past the queue.” That said, he did once tweet a picture of everyone waiting in line to get their photo taken. “Everyone was like, ‘Are you still there?’ And I was like, ‘Noooooooooo!”

Although the final Potter film, which let’s just say is a Big Deal for Neville, came out two years ago, the Potter phenomenon keeps on growing. “It’s sort of got, well, I say worse, but it’s not a bad thing. I get recognised a lot more nowadays than I did five years ago, which is odd.” Especially so when you consider that physical transformation: gone is the pudge, the lank hair, the buck teeth; and it’s not all down to training for Sunday’s 13.1 miles. And, yes, he did get his teeth “done”, which is basically shorthand for wearing braces. Not until he was through with being Neville, though; he was under contract not to fix his “pretty bad teeth” until filming had finished. “Which meant I had to go for 10 fucking years without having anything done.”

Even that didn’t satisfy the Longbottom aesthetic requirements, however. “When we got to film three, and I’d got quite tall and slimmed out a bit, they decided that they were uncomfortable with that, and thought, ‘No, we need to make him a bit geeky looking’, or geekier, I should say, so they put me in a fat suit and false teeth.” The whole ensemble made the poor teenage Lewis so miserable that David Yates, who directed the final four films, agreed he could ditch the teeth, although the suit stayed.

“Looking back on it, it doesn’t really bother me at all, but at the time, when you’re going through puberty and you’re on a film set with a lot of attractive girls anyway, and then back at school as well, and you have to wear this fat suit, and have this long greasy hair – oh, yeah, at the time, it was rubbish. It was really rubbish.”

It isn’t only the millions of Potter DVDs and endless tumblrs where the agony of Lewis as Longbottom lives on: in a glass cabinet in the study of his own parents’ house sits a plaster-cast model of Neville’s pug-eared head as it was in the second film. Complete with wig and Gryffindor scarf. There are also plenty of other Potter bits and bobs, but Lewis says his parents will kill him if he describes it “as a shrine one more time”. Oops. Too late. “It is. It literally is. I mean, it’s sweet, you know. But I think it’s a bit odd.”

He owes a lot to his mum; she had to accompany him during all those years of filming, when he did most of his growing up, along with the rest of the Potter crew. All those teenage crushes, like his pash for Emma Watson. “From 11 to 14 or 15. Absolutely. Obviously, I was a looker, so she missed out, clearly,” he grins. “You can put ‘sarcastic’ in brackets there. She’s a very, very good-looking girl, she was even back then. But I think everyone had a crush on someone. For 10 years!”

It’s his turn now, though, to be the object of lust, if the web is anything to go by. Lewis fans will be pleased to hear he is off to South Africa shortly to film his first big lead for the BBC, for a top-secret project that will air next spring. And on Friday, his latest film opens: a British heist movie called The Rise, starring Luke Treadaway and Timothy Spall. It’s an art-house This is England meets Ocean’s Eleven affair; gripping stuff. Ideally, for the Leeds-based Lewis, it was shot in his home town. He even helped his co-stars hone their accents over plenty of pub and football-watching sessions with his mates.

Not that you’d necessarily know it was Leeds. “It was supposed to be a nondescript northern town. But one of the things Rowan [Athale, the director] said was that he didn’t want it to be one of those typical knives, hoodies, guns, south London gangsta movies which he felt, and I agree, have been done to death. For a start, he didn’t want any guns in the movie … he didn’t want it to be the typical glamorising of that entire world.”

Perhaps handily for his post-Potter reinvention, the film also avoids glamorising Lewis, whom I initially failed to spot when I watched it. He will be hoping the crowds out cheering this morning for the Great North Run don’t repeat my mistake.