The only sin Scout Finch, the eight-year-old narrator of Harper Lee’s classic novel, has ever heard her lawyer father denounce was to kill a mockingbird; they do nothing but make music for us to enjoy.

The corruption of innocence is as much Lee’s theme as the rape and racism that disfigure a small Alabama town in the Depression, scene of Gregory Peck’s Oscar-winning turn as Atticus Finch in the movie, and now the setting in Regent’s Park for a quiet, shadowy but highly effective performance by Robert Sean Leonard.

Sean Leonard, who first (and last) appeared on the London stage in Thornton Wilder’s Our Town twenty years ago, and is best known as Hugh Laurie’s sidekick, Dr James Wilson, in the Fox television series, House, moves deliberately through the play (the authorised, standard adaptation by the late Christopher Sergel) in true Peck style, even down to the horn-rimmed specs and crumpled white suit.

In the great courtroom scene, where he’s defending a dignified Tom Robinson (Richie Campbell) on a trumped-up rape charge, he shows how the defendant couldn’t have throttled his alleged victim from behind by throwing him a glass to catch one-handed; the left hand is useless, mangled in a cotton gin years ago.

It’s a moment of stunning revelation in Timothy Sheader’s production, all the more so for being low key. In a theatre where Shakespeare has been the summer norm for decades, Sheader and his team are now finding a Shakespearean dimension, and reverberation, in modern classics like The Crucible and Lord of the Flies.

The actors materialise among the audience, each reading from a different edition of the book. This device both honours the source and suggests the childish wisdom in Scout’s narrative, and the actors are continuously referring their performances back to the text.

This lightness of touch throws Sean Leonard’s gravitas into relief, so that his saint-like Finch – derided on all sides for “lawing for niggers” -- acquires a rock-like stillness in a ferment of nattering.

At the same time, the work-obsessed widower is negotiating a new relationship with his own children, played with wide-eyed ingenuity (at the performance I saw) by Izzy Lee as Scout and Adam Scotland as Jem in denim boiler-suits.

Sheader’s production, played on a tilted, chalk board setting by Jon Bausor, is very good at maintaining this emotional three-way dynamic between community, children and the shades of fear and ignorance.

Every May the townspeople of Monroeville, Alabama, the home of Harper Lee, perform Christopher Sergel’s theatrical adaptation of Lee’s acclaimed, much beloved novel, on the grounds of the county courthouse. It’s a potent, somehow ironic demonstration of the enduring regard for the novel, in the very part of America whose racial intolerance Lee exposed.

It’s one thing performing, and seeing this story of the Deep South in its steamy locale, where even the shade is “sweltering” and “men’s stiff collars wilted by nine in the morning”, another outdoors in a still painfully chilly London May. But it’s testament to Timothy Sheader’s production that for long moments we are agreeably wrapped in the story’s magical embrace.

The play starts with a surprise. First one actress steps up in the midst of the audience and starts reading, literally, from a copy of the book. We assume this will be the narrator; but then another takes over, then an actor, and before we know it the six-year-old Jean Louise Finch, aka Scout, is multi-form amongst us.

While Sergel sensibly retained Scout’s distinctive voice by utilising an on-stage narrator, Sheader’s masterstroke is to share the task around his cast, all but four (the lead, and the children) dropping away from their characters to pick up a text and read to the audience. The move is not only theatrically dynamic, but somehow underlines the sense of ownership that anyone who has read the book feels for it.

The breathless excitement of the opening continues as the actors head for the stage – bare except for a tree – and start describing in chalk the Finch home and its immediate environs in the fictional town of Maycomb. Musician Phil King, who will regularly emerge with his guitar and harmonica, breaks into a song. And 1930s Alabama, bar the heat, is in place.

We are speedily, efficiently introduced to the chief characters. The tale centres on the Finches – Scout and her older brother Jem, their middle-aged, widowed father Atticus (Robert Sean Leonard, pictured above right) their black housekeeper Calpurnia and their friend Dill. The children’s mix of fascination and fear for the recluse Boo Radley informs many of their summer pranks; meanwhile Atticus prepares for the defence of Tom Robinson (Richie Campbell, pictured below), a black worker who has been falsely accused of rape yet whose guilty verdict is a foregone conclusion.

If at times we miss a true sense of locale (the accents seem middling amongst the adults, are understandably casual in the children), Lee’s themes are powerfully felt: the corrosive depiction of racism offset by her call for tolerance and empathy, the children’s treatment and changing perception of Boo Radley reflecting the bigger challenges faced by their father. It’s also perhaps the most affecting account of good parenting ever written. The scene here in which Atticus faces the angry mob intent on lynching Tom is appropriately chilling, his overwhelmed reaction to his children’s support in that moment – which effectively saves his life – and the moral he imparts as a result incredibly moving.

Much rests on the children. Making her professional debut, Izzy Lee captures Scout’s scamp-like quality (though perhaps overdoes the wistfulness), Adam Scotland Jem’s charming intention to grow up fast, Harry Bennett Dill’s desperate desire for a family. They’re an engaging bunch. But I was a little less convinced by the star in town – and the only American in the cast – Robert Sean Leonard.

While it would be unjust to evoke Gregory Peck’s performance in the flawless film adaptation, this does fall short of what I regard as Leonard’s potential for the role. The actor captures the character’s moral fibre, his paternal fondness, but somehow misses the inner strength. His courtroom summation is faltering, the bowed gait and melancholy air underwhelming throughout.

The ensemble is very effective, but special mention has to go to Simon Gregor’s villain of the piece, Bob Ewell, and Richie Campbell’s Robinson, whose courtroom speech captures the desperation of a man whose only crime is to feel sorry for a white girl.

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