AFTER making its regional debut at the Lowry in Salford, the National Theatre tour of The Curious Incident Of The Dog In The Night-Time moves on to Yorkshire on Tuesday.

Hull New Theatre will have first dibs, followed by the Grand Opera House, York, with further runs in Bradford, Leeds and Sheffield, such is the popularity of a West End hit that is on the road fromDecember 18 2014 to November 7 2015, visiting 31 cities and towns.

Adapted by Simon Stephens from Mark Haddon’s best-selling novel, Curious Incident charts the progress of Christopher Boone, aged 15, who stands besides Mrs Shears’ dead dog. Wellington has been speared with a garden fork and Christopher is under suspicion. He records each fact in a book he is writing to solve the mystery of who committed the murder. Christopher has an extraordinary brain and is exceptional at maths but ill-equipped to interpret everyday life. He has never ventured alone beyond the end of his road, he detests being touched and distrusts strangers, but his detective work, forbidden by his father, will take him on a frightening journey that will upturn his world.

The role of Christopher will be played by Swansea-born Joshua Jenkins in his National Theatre debut at 27, having performed already for the Royal Shakespeare Company and National Theatre of Scotland. Joining him on the British and Irish tour will be 26-year-old Leicester actor Chris Ashby, the alternate Christopher, who will share the weekly workload of such a physically demanding role. Initially, Joshua is doing all eight shows per week in Salford, before settling down to five, with Chris doing the other three.

"The audition process started off with just reading the script, reading a few scenes, but Round Three was a movement workshop, which was three hours of hell." recalls Joshua, speaking in the National Theatre rehearsal rooms in mid-November. "At the moment, we're doing a bootcamp; circuit training; one hour every morning; press-ups etc, and the rest of the morning is spent going through movements for the scenes."

Such preparation is vital, not only because of the length of the tour but also because each performance is so energy-sapping for all the company. Such physicality is necessary to explain Christopher's thought processes. "He's got the most beautiful mind you could imagine," says Joshua. "He has a wonderful talent for maths and he's extremely particular in everything he does."

Christopher is never called autistic in Haddon's book or Stephens's stage adaptation. "The way it was written, it's not about autism, just this boy who sees the world differently," says Chris Ashby. "It really does change your perspective of how brilliant that mind is when you try to learn primary numbers. Christopher knows thousands; we have to learn about 60 and we're really struggling."

Not surprisingly, Joshua says: "Playing Christopher is absolutely not like anything I've done before. My body has been stretched like I've never been stretched before and couldn't ever imagine being stretched like this! It becomes so much about getting into shape physically with a running time of two hours 40 minutes, with Christopher never leaving the stage."

Nutrition has to be considered too. "We had a chat with a nurse who says we need to eat 2,000 to 3,000 extra calories on performance days," says Chris. "It's a show that just doesn't stop."

All in a day's work! "Nothing puts you off," says Joshua. "Though you do think, 'what have I got myself into?', but it's unlike any other job a young actor could have and I feel incredibly privileged."

Chris concurs. "It's one of the few parts in contemporary theatre where it's such a beautiful play and yet so physically involving," he says.

Put yourself for a moment in Joshua or Chris's shoes. "On day two of rehearsals, Scott [choreographer Scott Graham], asked me to climb up a wall and I thought, 'you've got to be kidding. How do you do that?'," recalls Joshua. "But I managed to do it, and it's surprising what you can do when you stay open to the possibilities of what you can achieve."

On top of all that comes the demands of playing a 15-year-old boy when both actors are in their mid-20s. "It's all there in the script; you just have to go with it," says Joshua.

"Christopher may seem immature in some ways but at other times, he seems like a 40-year-old man in a 15-year-old boy," concludes Chris.

The Curious Incident Of The Dog In The Night-Time runs the Grand Opera House, York, from January 20 to 24; also Hull New Theatre, January 13 to 17; Leeds Grand Theatre, August 25 to 29. Box office: York, 0844 871 3024 or at atgtickets.com/york; Hull, 01482 300300 or hullcc.gov.uk/; Leeds, 0844 848 2700 or leedsgrandtheatre.com