Image by Brinkhoff Moegenburg

There’s nothing particularly curious as to why The National’s The Curious Incident of the Dog in the Night-Time has recently announced a renewed run at the Gielgud Theatre until 24 October. With seven Olivier Awards and now a sell-out production more than earning its keep on Broadway, Simon Stephens’ adaptation of Mark Haddon’s critically acclaimed book about an autistic mathematical genius is the gift that keeps on giving. But that doesn’t mean that Graham Butler, the actor playing protagonist Christopher Boone at the Gielgud, has started to relax too much, even though it’s now almost a year on since first picking up the phone and finding out he’d won the tremendously hard fought for part.

In fact, when I ring him for a chat about the renewed run, he’s only just finishing off being given notes on last night’s performance by Director Marianne Elliott – even though he’s now more than eight months into the role. “We don’t just rest and let it go stale,” explains Butler. “All of the physical movement has to be tailored to each new actor – there’s no set instructions – and I think that’s why it keeps things feeling constantly fresh”. It sounds like hard work but Butler disagrees – to an extent. “It is tough but I think it’s good to be reminded of the challenges – otherwise I think in the long run we’d be thinking “Oh I can’t wait to get home!” Instead I still feel really excited about coming to work.”

The excitement of everyone onstage in The Curious Incident is infectious throughout the performance, even when acting is only half their job. Many of the cast, including Butler, have to make intense moments of physicality seem like nothing. Since last May there’s been a strict exercise regime ranging from “sit-ups” to “squats”, which the “whole company do” and which helped ease everyone into the extremely active production. “I remember thinking at the beginning – well this feels exhausting!” admits Butler, “But like anything you begin to become immune to it, so now it feels relatively easy. I remember a friend saying “Oh it doesn’t seem that physical”, but that’s the point – we work that hard in order so that the audience aren’t just watching us move – they’re following the story.”

The narrative itself is one which has captivated thousands, with Stephens’ adaptation becoming so popular as to be added to the GCSE English literature schools set text, making it the only modern play currently performing in the West End to be part of the post-1914 drama and prose selection.

Butler remembers quite clearly where he was when he joined the success story – even though it was at the end of an audition process so predictably intense it’s surprising he remembers anything at all. “The final round was a movement audition and I was told “If you can get through this, you’ve got the part”. So I spent hours learning some of the choreography, and into the room came the Director and the Producer – who watched me sweating and panting away…When I eventually left it was late in the evening and I was making my way to the Globe Theatre. Just as I arrived I got a phone call saying “You’ve got the part”. But I couldn’t really celebrate – I just collapsed!”

Butler’s dedication in the auditions and the show itself partly illustrates the surprise huge popularity of the production and the enthusiasm to be a part of it – not just in the UK, but also now across the pond on Broadway, too. With its distinct British sense of humour it might have seemed a show unlikely to translate to our American cousins and beyond, yet in the several months that Butler has been performing as the boy with a gift for numbers and a problem with social norms, he’s been amazed by the huge variety of people who come to watch the performance. “I think there’s just something about this play and the original book that’s really universal” agrees Butler. “Being in the West End we have the privilege of getting people from all over the world in to watch and generally the same positive reaction is had by all. It’s such an amazing reaction to have a play in the West End be so adored.”

Indeed, as the new run confirms, even when a theatre’s actual architecture threatens to physically shut down proceedings, there seems to be nothing that can stop Mark Haddon and Simon Stephen’s flyaway success. As for Butler, he won’t be relaxing even in-between performances. The actor is also learning how to play a zombie for the second series of Sky’s A Penny Dreadful – a whole new kind of physicality just waiting to be learnt.

The Curious Incident is playing The Gielgud Theatre until 24 October 2015. For more information and tickets, see here.