By Michael Osborn
Entertainment reporter, BBC News
Brothers David and Ben endure a fraught relationship in the drama
The debut TV drama from award-winning novelist Mark Haddon deals with the dark side of teen angst and a complex sibling rivalry between a boy and his Down's Syndrome brother.
His Whitbread Award winner The Curious Incident of the Dog in the Night-Time, which sold more than one million copies, is narrated through the eyes of an autistic 15-year-old.
Haddon says the inspiration for writing Coming Down The Mountain was to get co-star Tommy Jessop on screen - the first Down's Syndrome actor to play a leading role in a TV drama.
"I wanted him to be in people's living rooms for 80 minutes in the evening and to spend some time with him," he says of Jessop, whose portrayal of Ben gradually transforms as the drama progresses.
"At first we see Ben through David's eyes, but he gradually comes to life, gets space to talk and takes over the screen. He is a 'potato' to start with, but by the end of the film, takes it over," explains Haddon.
"This is the biggest and most moving thing about it.
"Making his character realistic was no different to any other. This is the kind of writing I like to do as often as I can, which will prompt half the audience to react in one way, half the other.
The play's power lies in the the central relationship between brothers Ben and David, with the younger sibling ultimately pushed to take drastic action.
"It's also story of younger brother David, about being a teenager and hating your brother, which everyone has been through," says the author.
David sees his budding romance scuppered by his brother
Actor Nicholas Hoult, who appeared in the film About A Boy and teen drama Skins, says playing out moments of abuse with his co-star was difficult.
"We didn't know each other at well at first and it was quite frightening, but we talked through scenes and I made sure I wasn't pushing him too hard.
The 17-year-old says script was "not like anything he had read before."
"It was completely different to everything I'd ever done before, given the relationship between the two brothers. I didn't know a lot about Down's Syndrome, so I had to learn as I went along," adds the actor.
As well as David's intensely difficult relationship with his brother, the drama explores his own adolescent growing pains - which are compounded when the family moves away from London to allow Ben to attend a special school.
"He is growing up and experiences his first love. When they move it's the end of the world and makes him even madder," says Hoult.
Jessop, 22, praised by the makers for his "phenomenal" script-learning skills, was picked from some 130 actors and "really shone out".
"I did the best performance I could - I'm confident as an actor. It was quite upsetting to do the shouting scenes. Me and Ben have got the same kind of life - but I never get shouted at," he says.
Producer Roanna Benn says: "Ben is just a character in the drama who has a strong personality, has a sense of humour and is a rounded individual.
"He stands up to his brother and takes control of the situation, which Tommy could really shoulder," she adds.
Coming Down The Mountain will be shown on BBC One on Sunday 2 September.