Meeting Robert Sean Leonard, the mature but still fresh-faced actor who first came to prominence in the 1989 film Dead Poets Society and has spent the last eight years co-starring in House, opposite Hugh Laurie, is to find an actor not distracted by the pursuit of fame and celebrity, and totally honest about the business he works in.

Now aged 44, he’s been in it for a long time – he first began as a child actor on the New York stage, both off-Broadway and on, and first came to London 22 years ago to star in the West End in a production of Thornton Wilder’s Our Town. “I was a kid when I was here, I was 22 or 23. There are some things that don’t feel long ago – I met my wife 18 years ago, and to be honest it feels like maybe five. But Our Town was 22 years ago and that feels about right to me. It feels like a different world.”

Not that he doesn’t have fond memories of that time: “Mostly it’s a play that I adored and I still do, and so it was a pleasure to do it here. It hadn’t been done here since the original in 1946. And a big part to me of what makes theatre exciting is bringing it to a new audience. Sometimes I’ll have a picture of the playwright with me, as I did when I did Long Day’s Journey into Night on Broadway; others listen to Bon Jovi to get inspired, God only knows, but when you’re doing a show eight times a week for several months, what never fails to get me excited is hearing the audience over the tannoy, and thinking, especially if it is a play you admire, that there are probably 900 people who have never seen this play before. When I did Pygmalion recently in San Diego, I felt George Bernard Shaw was somewhere looking down and saying to me, ‘You’re on – and these people may never see Pygmalion again’. It gets you excited.”

He has, despite his more recent run on television and the fact that he and his family are now based in California, remained faithful to the theatre more than anything else, clocking up an impressive dozen Broadway appearances, including a Tony for best featured actor when he starred in Tom Stoppard’s The Invention of Love in 2001, and two more nominations for Candida and Long Day’s Journey into Night. When I say how glad I am that he prioritises theatre, he says: “Sure, but that’s like admiring someone for being faithful to Donna Reed – why wouldn’t you be?”

And then he reveals something very frank about the distractions of television and film: “There’s a reason for me that they pay so well, because it’s such a drag – there are too many hours, and I don’t enjoy it very much.

“When I was a kid it was a little bit exciting working with Peter Weir and Robin Williams, but that faded pretty quickly for me. I probably haven’t even seen ten of the films I’ve done. I don’t get a joy out of it, and I don’t go to the movies. The last film my wife and I saw in a theatre was March of the Penguins in around 2005. It’s just not a part of my life.”

Working in television for so long, he said, had also proved to be a “lonely” job. Not that it’s a criticism of the company he was working with: “Everyone on the show was perfectly lovely and I never had a problem with anyone, but in one day of rehearsing here in London with these kids and people here I’ve had more connection and warmth than I had there the whole run. I’m not nervous to say it. You go in, you do your scenes, and you hope you wrap early so you can go home again. It’s not like sitting down and putting your make-up on and hearing the audience arriving. It’s so different, I almost can’t compare them.”

Nonetheless, he says, “I was proud of House, MD, and I think it was good for what it was and remarkably smart for a television show. I loved Hugh Laurie and was proud to be by his side that long and to be trusted by him. I enjoyed seeing him every day – he’s an incredibly good and talented and smart guy, and if you’re going to be stuck in a room for eight years with someone, believe me, you could do a lot worse than Hugh. But it’s not my thing – it’s not why I do this. And it’s certainly not what I do best by a long shot. I’m not being falsely modest. It’s just not what I excel at. I think I’m fine, but there are people who can do it much better than me.

“I’ve also seen Mark Rylance and people who blow me out of my seat onstage, and I feel I’m up there with them. Maybe not for every show or every role, but I know I can do it – I can hit it out of the park. And I don’t think I’m capable of it in film. I couldn’t hit a home run there. Susan Sarandon is good at it, Sam Shepard is good at it, Meryl Streep is good at it – I’m just okay.”

And right now, he is literally hitting it out of the park, returning to London to star in a stage version of Harper Lee’s To Kill a Mockingbird at the Open Air Theatre, Regent’s Park, directed by Timothy Sheader. He first ‘met’ him when he sent a video audition for Sheader’s New York recreation of his Regent’s Park production of Into the Woods last summer. “I’d been in the original workshop of Into the Woods 700 years ago, when I played Jack, so I tried very hard to get into that – I wanted to play the Baker. I didn’t get it, but then this came up after Tim had lunch with my agent David Kolodner, who is a very big theatre guy.”

Leonard, of course, had his own doubts, and true to just how frank he is, he shares them. He was keen to come back to the theatre and appear in London, but “the job itself I didn’t know, and I still don’t know”, he says, talking during the early days of the rehearsal process. “My first question was why? It’s a great book, but so is Moby Dick. I’d never heard of this play being done in the States, and didn’t even know there was one, but here it is done quite often. But Tim explained his great idea for it, which I won’t spoil by sharing with you. I think people will really respond to it – or it could crash into the mountain.”

He had another doubt: “Wrestling with the ghost of Gregory Peck in front of 1,200 people every night wasn’t exactly exciting to me, either. It could be a complete disaster. But my wife, because she’s a genius, said to me, ‘You know what, yes, it could. But your daughters have never seen London, so even if it’s a miserable failure, it’s only for two months’.

“So it fell very strongly into the ‘why not?’ category and I wasn’t going to lose. And the longer I am here and the longer I work, the more positive I’m getting.”

He’s clearly an actor who thinks deeply about what he is doing. And he also weighs up the rewards and dangers of his profession carefully.

“It’s a strange job, I’ve seen a lot of bad things come of it to too many people, but I’ve seen a lot of good things too. Why you get into it is important and what you inherently want. I wanted to be in the sandbox with the people who excited me. My greatest desire was to be in a sandbox with Kevin Kline or Kenneth Branagh – to be with the people I admired, and I have. I don’t think that’s always the driving force for a lot of people in my profession – there are sometimes darker All About Eve things about attention and need and fulfilment. I thank God that isn’t what drew me into acting, because it isn’t there. That’s your family and daughters and dog and fireplace. You don’t get that kind of comfort from acting – it doesn’t validate you in the way that some people hope it will. My wife and my daughter and my dogs validate me, not a Tony award.”