Mark Haddon: First he tackled Asperger's, now the writer is putting Down's syndrome in the spotlight with a new drama

"Last summer, I decided to kill my brother." Coming Down the Mountain, the first TV drama by Mark Haddon, opens with this provocative line.

But the controversial nature of the film does not end there. David (played by Nicholas Hoult), the putative brother-killer, is in a rage because he feels his life has been ruined by his sibling, Ben (Tommy Jessop), who has Down's syndrome. David is brutally disdainful of Ben: "Sometimes they change the recipe and you get a brother who is a big potato with eye tentacles. And then you might as well talk to the dog – if you have one."

Later, a livid David rails at Ben: "Why are you so clumsy and stupid and pathetic? Why do I have to take responsibility for you all the time? You're not my brother." At the screening I attended, there were sharp intakes of breath all around the auditorium at the harshness of that line.

David's fratricidal feelings are exacerbated when he is forced to move with his family from trendy London (and his gorgeous new girlfriend, Gail) to deepest, darkest Derbyshire so Ben can attend a more suitable school.

David vents his fury on his hapless father (Neil Dudgeon): " Everything's always about Ben. Ben is the reason why I don't have any friends and why I'm not with Gail anymore. Everything is so Ben can have any easier life. Ben wants a Ferrari and a Swedish masseuse. I'm sick of bloody everything about Ben." These are certainly not lines transcribed from the Manual of Political Correctness.

Then David hatches a fiendish plan to take Ben up a mountain and dispose of him once and for all. But – without giving the game away too much – Ben emerges in a new light. He displays a perspicacity and a wicked sense of humour that he had previously been too cowed to exhibit. We realise that – like David – we have fallen victim to prejudices about Ben and underestimated him.

Haddon has made his name taking on such difficult subjects. His novel, The Curious Incident of the Dog in the Night-Time, about a boy with Asperger's syndrome, became a runaway best-seller. The author, a charismatic man who twinkles like the earring in his left lobe, is well aware that David's attacks on Ben might make some viewers squirm.

The writer remains unrepentant, however. "I'm sure some people will get the wrong end of the stick about the scenes where David abuses Ben," admits the 44-year-old. "But no writer can control the response of an audience. You make a film you feel is as real as possible and hope people react as though it were real. The way of creating believable characters is not by conforming to a set of PC rules. I don't want it to be sanitised."

The author adds that he worked for some time with people with Down's syndrome when he left university, so he feels he knows what he is talking about in his portrayal of Ben. "If you're going to write something dark and funny about disability, you have to feel comfortable with your subject. And the fact that most of the audience will feel uncomfortable helps. Things can be funny when people are uneasy. It softens them up and stops them falling asleep on the sofa. I like those moments where people half-smile and half-wince."

Even though Coming Down the Mountain has been made with help from the Down's syndrome Association, Haddon is prepared for criticism. "I hope that people with Down's syndrome will be chuffed with this film. There is something validating about seeing people like yourself on screen.

"But some people will doubtless hate it. If people like you are portrayed very rarely, then the expectation is that the character will represent all of you. There are so few characters with Down's syndrome in dramas that when one comes along, he or she carries a heavy burden. It's like being the only gay man in EastEnders a few years ago."

Roanna Benn, the producer of the drama, which goes out on BBC1 this Sunday, also trusts that audiences won't misunderstand Coming Down the Mountain. "Of course the film is not saying people should behave like David does towards Ben. Are people laughing at Ben? Absolutely not. Part of Ben's personality is that he says these very funny things – he has this tremendous spirit. He's a fully rounded human being."

Haddon reckons that is reflected in the actor, too. "What makes Tommy Tommy is his sense of humour. Down's syndrome is such a small part of who and what he is."

In person, Jessop is a vibrant, magnetic presence. When asked what was the hardest challenge he faced during the production, he responds, quick as a flash: "We had to re-shoot one scene several times, and I had to eat eight Weetabixes, 10 cookies and five ice creams. I'm not going to do that again in a hurry!"

Jessop, 22, who was selected for his debut TV role from 130 actors with Down's syndrome, is also well capable of looking after himself. "Was I concerned about being horrible to Tommy?" Hoult asks. "No, he can take care of himself!"

Haddon believes that viewers will gain something from listening to a voice like Jessop's that is all too rarely heard in British TV drama. "We need to see more of Tommy and people like him on screen," the writer says. "Hearing his voice gives me a terrific frisson. I wanted Tommy to be in people's living rooms for 90 minutes on a Sunday night."

Haddon, the father of two young sons who, he hastens to add, are nothing like David, sighs that people with disabilities are often dramatised in a condescending manner. "A lot of roles for people with disabilities are quite patronising. It's a real pity when they are just used to give dull PC kudos to a drama, or when they're wheeled on in a tokenistic way without any real involvement in the plot. I wanted Ben to drive the story. Two-thirds of the way through Coming Down the Mountain, he suddenly grabs the steering-wheel and won't let go."

The writer says that actors with disabilities are also often employed as "catalysts to elicit interesting reactions from "normal" people. "In Flesh and Blood, for instance, we never get inside the disabled characters' heads – their role is simply to cause a profound impact on Christopher Eccleston's able-bodied character. I think it's more interesting to take it a stage further and ask, 'what does the world look like from their point of view?'"

Haddon, whose previous forays into TV include an adaptation of Raymond Briggs' Fungus the Bogeyman and the comedy series, Microsoap, is keen to stress that Coming Down the Mountain is not bound by the curse which afflicts so many dramas about people with disabilities: a stifling sense of worthiness.

"If you set out to be didactic, you can kill a piece," the writer comments. "My hope is that there will be viewers who have never thought about people like Tommy before. If they're obliged so spend 90 minutes in his company, it'll be hard for them not to come out of it slightly changed."

However, Haddon is anxious that Coming Down the Mountain is not consigned to the dreaded pigeonhole marked "issue drama". "I think that would belittle this story," the author says. "It's not just a film about Down's syndrome. It's about the way in which a family can be thrown by the different needs of different children. That's a state common to many families. These characters may seem odd at first, but I think they come alive when you find aspects of them that chime with your own life."

The writer, who says he has a very good relationship with his sister, adds that, "the film is also about intense sibling rivalry. Unless you're a single child, you can understand how homicidal sibling rivalry can be. Sibling rivalry goes back to Cain and Abel and beyond. Children are competitive beings – they don't want to accept 50 per cent of their parents' interest. Even people who love their siblings have times where they want to strangle them."

The film is also an acute portrayal of teenage alienation. "Anyone can relate to David," Haddon observes. "It's not as if being an angst-ridden teenager is unusual. If you don't feel anxiety and depression as a teenager, then some of your circuits are missing. We all have difficult childhoods in some way because we haven't yet learnt how to deal with the crap of life."

Haddon concludes with a fervent hope that the film will help further Jessop's career. "If lots of producers see Coming Down the Mountain and think, 'let's give Tommy Jessop a ring', that would warm my heart. It would be great to see him doing a part that didn't have much to do with disability at all."

Jessop is equally hopeful that Coming Down the Mountain will trigger more opportunities for him and other actors with disabilities. "Are doors opening for me? Probably," he says with a smile. "I really want to be a full-time actor – who wouldn't? My favourite actors are Tom Cruise, Johnny Depp, Orlando Bloom and Brad Pitt – some people even say I look like Brad Pitt."

Finally, what kind of role would Jessop like to play next? Without a moment's hesitation, he replies: "Heart-throb."

'Coming Down the Mountain' is on BBC1 at 9pm on Sunday