Having weighed it up in my head, I decide not to tell Matthew Lewis that I have never seen any of the Harry Potter films, in which he appeared as Neville Longbottom for a decade. You never know how someone who has been part of one of the biggest blockbusters of our time will receive such a confession. On reflection, however, I suspect Lewis may have welcomed it. He may even have been relieved.

Because, as he explains when discussing life after Harry Potter, he has had to work hard to prove to those who do know the films that he’s more than just the child actor who appeared alongside Daniel Radcliffe.

“I am fortunate that I get sent scripts and get to meet people I would never have met had I not done Harry Potter,” he admits. “But I feel I had to come out of that show and prove that I am not a one trick pony and can do other stuff. I have to start at the bottom really. I am fortunate that it opened doors but now I have to step through them.”

He adds: “I did feel immediately after it, when I went to auditions, that the worst thing I could hear was the director saying, ‘I am a big Harry Potter fan’, as immediately you think, shit, I have to convince them I am not Neville Longbottom before I can convince them of anything else. It was the hardest thing. But since then, I have realised you can’t think negatively. You do the best you can do and if it’s what they’re after it’s what they’re after, and if it’s not it’s not. Simple as that. And you’ve just got to get on with it.”

In actual fact, Lewis has already begun to prove himself since Potter. Most recently he has appeared on television in the BBC series The Syndicate, which was his first, what he calls, post-Potter TV project (I did see him in this and tell him so). The drama was written by Kay Mellor, who, it transpires, actually gave Lewis his first TV job when he was a small boy. And last year Lewis made his stage debut in a touring production of Agatha Christie’s Verdict.

Now, he is starring in the West End, in a play called Our Boys, by Jonathan Lewis (no relation by the way).

The play is Lewis’ West End debut, but he admits that, following his stint in Verdict, he was a little reluctant to do any more theatre.

“To be honest I was not interested in doing anything again so quickly after [Verdict],” he says. “But then my agent gave me a call and said there was this play called Our Boys and asked if I would like to go in for a meeting about it. I was apprehensive, given it was the West End, as I was not sure I was ready to go and do that. But then I read the script, and it’s one of those scripts that you can immediately picture and see on stage. I thought, ‘Okay, I want to go into this. I doubt they will cast me but it will be a good laugh to go in for it’.”

I press him on why he was not very keen to do any more theatre after Verdict. Was it such a bad experience?

“To be fair, when I started doing Verdict I literally had no idea what I was doing,” Lewis, whose career has been so screen-based, admits. “I wanted to do some theatre as I wanted to do something different. I wanted to learn and get an understanding of the craft. But I jumped in at the deep end and it took a long while to pick stuff up and learn. And I did learn a hell of a lot on that gig.

“With Verdict I was away for six months, touring, with a week in each town. I did not really enjoy that aspect of it. But with Our Boys being such a great play, and in London, so I know where I am for a 12-week run, I was much happier to do it. And I really am enjoying it this time round. I feel I am much more rooted and know what I am doing a bit more.”

Our Boys, directed by David Grindley, is based on Jonathan Lewis’ own experiences of being a soldier in hospital. It is described as a “comic and moving testament to the men Jonathan shared his hospital bay with”.

Lewis describes his character, Mick, as a “really good guy”.

“He loves the army, it’s his life,” he says. “He loves everything about it – the discipline, the uniform. And he’s not afraid to stand up for it when it comes into question.”

Even on Harry Potter we did not rehearse. So it’s nice to go into a rehearsal room and just play around with ideas in front of a director

Lewis goes on to explain that Mick is in hospital for a circumcision.

“I’ve been told by several soldiers we have had in that this is surprisingly common in the armed forces, because, when you are out on operations and only showering once a week, infections can incur. So it’s better for the army to have it off,” he says.

Lewis adds that during the course of the show’s rehearsal, the cast and crew were visited by real-life soldiers, and the playwright himself, who of course experienced the army first hand.

Lewis describes having the writer around as a “strange pressure”, but useful in terms of being able to quiz him on aspects of the drama, which is set in the 1980s during some of the worst periods of the Northern Ireland conflict.

“Apart from the Falklands, that area of history was a peacetime army,” Lewis says. “It was a good time to be in the army, but on the flip side, there was the troubles and threat from the IRA. The soldiers would not go outside in military dress, as the IRA was targeting soldiers in the street.”

Our Boys also stars Laurence Fox and Arthur Darvill. Lewis says he has enjoyed the rehearsal period with the other members of the cast, admitting that rehearsing is an alien concept to him, given his background in film.

“You don’t get time to rehearse in film and TV at all,” he says. “Even on Potter, where we had this huge budget and time, we did not rehearse. So it’s nice to go into a rehearsal room and just play around with ideas in front of a director. And you get to create a fuller character, which I am really enjoying. Every time I do a new job I learn a little bit more.”

In actual fact Lewis has been learning since he was five years old.

His interest in acting was sparked after seeing his older brother do it, and following him around Yorkshire, where he was raised, on various jobs.

He says he just assumed that this is what everyone did for a living, and so he begged his brother’s agent to sign him up too.

“Eventually she was so sick of me nagging, she said, ‘I will send you to an audition and if you get good feedback, when you turn six you can join me’. She sent me off, and I got the part, so she let me in after that.”

Every job I do I find myself sitting quietly watching the other guys, trying to pick up as much as possible

The part was in the aforementioned 1995 Kay Mellor drama called Some Kind of Life, with Jane Horrocks. Lewis has been working ever since, landing the part of Neville in Harry Potter in 2001 – which kept him employed for the next decade. And he has been learning on the job the whole time.

“Every job I do I find myself sitting quietly watching the other guys, trying to pick up as much as possible,” he says. Lewis adds that, despite not having had a formal training, he wouldn’t go to drama school now. He says he would be concerned about how what he has learnt this far would fit in with a formal training.

That said, he knows someone at Manchester School of Acting, whom he plans to visit next year for what he calls “bespoke” acting classes, to help him keep things “fresh”.

And what of future work offers? Does he find himself being particularly choosy about parts that come his way now?

“I have turned down some stuff,” he admits. “If I don’t think I will enjoy something I won’t do it just to make money. I want to enjoy what I am doing.”

He adds: “If I am going to be away from home so long, then I want to make sure I am not depressed. Because if you’re not happy with what you’re doing you will only give a shit performance.”

Our Boys runs at the Duchess Theatre, London, until December 15