Robert Sean Leonard has just dropped his daughter off at school, albeit with some difficulty – the little Leonard didn’t want to be left and now her father feels traumatized by the experience. Family, you see, is Robert’s thing; more than acting, more than art and very much more than celebrity.

That’s not to take away from his talent or commitment to his craft: Leonard has been treading the boards since the age of 13, when he caught the acting bug through his mother, who painted signs for the local amateur dramatics club.

“It was all rather amateur but I tell in love with the people,” he tells me, “Like when you first hang out with people outside your family and are completely won over.”

The theatre may be Leonard’s first love, but television and film work soon followed. His movie debut came via Dead Poets Society, a role that earned him a cult following.

“I’ve not been in many films,” he says, “I went to auditions but never expected to get callbacks. River [Phenix] and Keanu [Reeves] got the callbacks. [But Dead Poets Society director] Peter Weir wanted unknown actors so he could really build the story of the schoolboys without the audience focusing on a main character.”

A moment’s pause, then: “Thank God, he didn’t want River”.

A few minutes into our transatlantic phone call and already it’s clear that self-deprecation, frankness and wit are Leonard hallmarks; they make discussing the movie business much more fun. I ask what it was like to work with Winona Ryder and Daniel Day Lewis in The Age of Innocence, and the anecdotes fly.

“I still didn’t know how that happened,” Leonard says, then: “Actually I do, you’ve jogged my memory. Winona Ryder wanted me to play her husband in Bram Stoker’s Drakula. We went to the premiere of Great Balls of Fire together – she’s since said that she started dating Johnny Depp that night and I’m literally like, “Where was I? I must have been chatting to Kevin Kline in the corner”. Anyway, I didn’t get the part – Francis Ford Coppola must have humoured me – but Winona then championed me for The Age of Innocence and I didn’t even have to audition.”

I venture That Winona Ryder must have been pretty impressed by him, but Leonard plays the connection down.

“Oh. We were kids. It was like the “40s or something; we started the same agent so we went out together. They pair you up like that as if you might be dating or there’s the possibility you might end up dating”.

We go on to talk about Robert’s involvement in TV’s House, where he played Wilson. Best buddy of Hugh Laurie’s eponymous doctor, for an impressive eight years. The series accounts for his longest-running role. So I can’t help but wonder what he thinks it’s done for his career – and once again, he proves himself to be very uh-Hollywood and unable to believe the hype.

“Who knows… no one really knows that stuff. Would I have got other roles from it? Has Tim Sheader [director of the Regent’s Park Open Air Theater’ cast me as Atticus Finch because of it? Probably. House has been very helpful to me because theater doesn’t earn much money – in fact, I/ll be working on a loss once I’ve flown everyone over to London to see the play! House means I can send my daughters to college. It was an easy gig; I liked playing the sidekick of the main guy. Hugh worked so hard on the show and promoting it: I’d get to be home while he was, y’know, sat at the MTV Awards”.

His part in Sheader’s production of To Kill a Mockingbird means that Leonard is about to fly to Britain for a one month stint at the Regent’s Park Open Air Theatre. I ask if he’ll be seeing his old colleague while he’s in town.

“Oh yes, Hugh and I get on famously. I love Hugh. I’m a bit of an anglophile so I knew all about his work beforehand; with Kenneth Branagh – who I stalked for the part in Much Ado about Nothing – and with Stephen Fry and Emma Thompson”.

How about the rest of the House cast, did everyone gel as well as Leonard and Laurie?

“We heard there’d been a punch-up or something on Grey’s Anatomy; nothing like that would happen on House. We were all like, how would that even occur while you were filming? We had so much fun”.

Despite his forthcoming sojourn on British shores, I’ve read that Leonard hates travelling. Can he really be an anglophile?

“I’m an anglophile from my couch,” he laughs. “I never said I wanted to fly anywhere! I get anxious about flying. Once I get to a place I’m okay, but it’s the thought of the unknown. My wife is the opposite, her dream is to go to India and stay in a rundown hotel like in that film The Best Exotic Marigold Hotel. I’m not very adventurous. I don’t know why – my family didn’t travel, so maybe I’m not used to it. I didn’t get wanderlust”.

It’s not something he’ll let stand in the way to good role. Through, and our talk turns serious when we touch on To Kill a Mockingbird. Leonard has been working on adding a southern twang to his voice in preparation for his starring role as Alabama lawyer Atticus Finch, maintaining that, although the accent “might not to be really strong, it will certainly be there”.

He’s also revisited Harper Lee’s classic novel many times over in recent months, and his respect for the source material is clear.

“It’s such a seminal book for many people and the role of Atticus Finch is one that is held dear. I’m so glad Harper Lee only wrote one fictional book, which adds to the mystery of its greatness. I’m not looking to explore the dark side of Atticus Finch or the “emotional underbelly” of this man… I think his omniscient presence is there for a reason. Even if he had cracks, he wouldn’t show them to his children or to the world”.

This measures approach strikes me as being completely of a piece with the man behind it: Leonard doesn’t seem the type to overact. Instead, he allows his own lack of ego to lead him to a character’s central psychological truth. He does, However, confess to feeling daunted by the part, adding that: the fear has been eating away at me for months. I went to the Twentieth Century Fox warehouse to get clothes because I thought: I can’t turn up to rehearsals in Reeboks, jeans and a t-shirt, it’s too big a leap. I’m not going Daniel Day Lewis on you, but I have to dress up for Atticus”.

On that note, I ask if there are any other actors whose careers he admires. His reply is immediate.

“at 20, I might have had a ready answer to that, but the older you get, the more you see the flaws: there’s a hole in even the prettiest life. There are people whose work I admire – Sam Waterstone, Mark Rylance – but I don’t think on that too much”.

And why should he, with his own future plans providing much to look forward to? Later this year, Leonard will play Henry Higgins in a US production of George Bernard Shaw’s Pigmalion, and he’s hopeful that he and (ex-ER star) Noah Wyle will reunite for further episodes of acclaimed TV drama Falling Skies. Until then, however, the good stuff is all ours: Robert Sean Leonard is London-bound.

To Kill a Mockingbird runs from 16 May – 15 June at Regent’s Park Open Air Theatre