Robert Sean Leonard is just out of rehearsals for To Kill A Mockingbird but already he’s on his feet recreating a scene from Harper Lee’s 1960 Pulitzer-winning portrait of Deep South racism.

It’s the moment when Atticus Finch cross-examines Mayella Ewell, who has cried rape against a black man.‘Atticus says: ‘‘I have nothing but pity for the witness but my pity does not extend to putting another man’s life at risk,’’’ says Leonard, all crumpled hair and rumpled suit.

‘He hates that he has to do this. In the book, there’s a line that goes: ‘‘Atticus didn’t much care for criminal law,’’ and so that’s what I learned about Atticus today. Tomorrow there’ll be something else.’

He looks elated and exhausted. He is clearly happier recreating the play than talking about it.

Leonard, who you probably know as Dr James Wilson in TV medical drama House (and, if your memory stretches back that far, from Dead Poets Society) is in town for the Regent’s Park Open Air Theatre’s new production of Lee’s classic novel. Normally an actor would wax lyrical about the project or, with material this iconic, discuss at length how relevant it is. Not Leonard.

‘To be honest, I’m not sure why I’m doing it,’ he says. ‘A friend said: ‘‘So you’re going to wrestle the ghost of Gregory Peck [who starred in the 1962 film] in front of 1,200 people every night?” and I though, what’s the point? It’s a terrible idea.’

Does he not think the play is important? ‘I’m allergic to the word “important” in film and theatre. Cancer research is important. Don’t get me wrong, I treasure the book. But I’m with Paul Giamatti on this. He once said to me: ‘‘I’m 38 years old and a huge fan of cinema but I’ve never once seen a movie that’s important.’’’

Leonard doesn’t do platitudes. He’s charming but he’s also frank. He knows the success of House, in which he starred opposite Hugh Laurie’s maverick medic, is why he’s here but that doesn’t mean he owes the show a debt of loyalty.

‘I would start work when the sun came up and finish when the sun went down, and all I got to say was: ‘‘Seen Cuddy’s ass today?’’’ he says. ‘It wasn’t digging coal but the gulf between these great TV shows and what it’s really like making them is unfathomable.’ And he’s candid about why he signed up for the Fox drama in the first place: ‘Money. It’s why I can afford to spend the summer in London.’

Unlike many screen actors who nip on to the stage every ten years to boost their credibility, Leonard can’t be accused of career opportunism. He’s worked on and off Broadway since leaving school at 16 and moving to New York by himself. In that time he’s built up a solid back catalogue in plays by Tom Stoppard, Eugene O’Neill and his favourite, George Bernard Shaw.

He set up a guerrilla theatre company with Ethan Hawke and this year played Henry Higgins in Pygmalion in San Diego.

‘As a kid, the theatre always felt a bit like running away to join the circus,’ Leonard says. ‘I remember seeing Kevin Kline in Shaw’s Arms And The Man when I was about 14 and feeling the hairs go up on the back of my neck. You never get that in TV.’

Being a household face pleases him – ‘it’s very human to want to be recognised for your work,’ he says – but Leonard is far from an industry player.

‘I remember watching Gwyneth Paltrow deliver her Oscar acceptance speech in floods of tears,’ he says. ‘My God, how open. The poor girl was just so intimate with these unnamed millions of people. I couldn’t do that. I just feel like a squirrel at a fox hunt in those things: so awkward and uncomfortable.’

Perhaps that’s why he’s opted to spend a possibly rain-soaked summer on a London stage: it affords him a degree of anonymity. And magic. ‘I just love those hair-on-the-neck moments when things are possible,’ he says. ‘That’s where I want to be.’

To Kill A Mockingbird previews from tonight, opens next Wed and runs until Jun 15, Open Air Theatre Regent’s Park. www.openairtheatre.com