Show me a clown, and I’ll show you a damaged child. This sad old truth has seldom been better exemplified than by the tumultuous king of impro-comedy, Mike McShane.

After spending the first 20 years of his professional life exorcising early sadnesses, McShane has now tackled the trauma which almost finished him for good. The resulting film is a junior version - though much stranger and sadder - of Milos Forman’s One Flew over the Cuckoo’s Nest.

The key real-life event took place in his native Kansas, and this is how he now recalls it. “One day - I was just 16 my mum and dad said we were going out for a drive. And they said they’d packed some clothes for me, and I remember thinking, what’s going on? But we drove past the turning to the place they said we were heading for, and suddenly took the turning for a place called Osawatomie.

“And I went, ‘No, No!’, because I knew what Osawatomie was - everybody did, it was the local state mental home! I started crying, helpless - should I grab the wheel, or get out and run? - eventually I just collapsed, resigned.”

It had been decreed that he should spend six months in this grim institution. “Half the people there should never have been in mental hospital, and half were so far gone, they were just shells. Some had committed murder. Some were like me, a bit mixed up.” He passed every day in a huge room with 100 others, forbidden matches or anything which could cut or maim, watched over by staff from the shelter of a glass booth.

To which we should add some earlier history. Adopted in infancy by a straight-laced Catholic couple, McShane had always been, as he puts it, “operatically emotional”, and he once greatly perturbed his parents by dressing in rags and begging from neighbours outside the local superstore.

“But as I got into adolescence, I got really out of hand.” He was first sent to a Jungian analyst, but when the man decided - correctly - that the problem was to do with family relationships, his affronted foster-parents aborted the treatment. “They sent me instead to a doctor who was the epitome of all that was wrong with psychiatry. A tiny, neatly groomed man who sat in a darkened room with the light coming into my eyes, asking me questions.” This doctor put him on drug therapy - and then engineered his incarceration.

When McShane managed to escape back to “normal” life, he gravitated to acting, and one of his first big parts was as the Indian chief in a stage version of One Flew over the Cuckoo’s Nest - in which an angry patient foments rebellion in a mental home. “But I loved the play too much. I wasn’t good in the role, because I used it as a platform. That was an important lesson - if you’ve got an axe to grind, create your own vehicle for it.”

Which, with vigorous encouragement from the BBC, he has now done. In Crazy for a Kiss, soon to be broadcast in Britain and released as a feature film in America, he plays a clumsily good-natured house father, one of whose charges is a tormented character based on the young McShane.

Apart from this direct correspondence, Crazy for a Kiss juggles the elements of his personal calvary into something more like the world of entertainment. As, he concedes, it had to. “My mind is very black, very paranoid, very furtive, and it always was.” It was put together with script-writer Greg Snow; they talked for a day then Snow went off to write. The script went through seven major drafts.

“Mike’s been very generous, in giving his story to me,” says Snow, who has drawn on his own family traumas for inspiration. “The script was as much about me as it is about him. This film is like handing on a garment - and when you hand it on, you have to let people wear it as they wish. There was a film in my head, but when I first saw the rushes I realised that it was now a film inside the director’s head.”

Director Chris Bould has striven to give the story an implicitly polemical edge. “It’s to do with teenage Angst. None of the kids in this story should be in a loony-bin - there, but for the grace of God, go any of us, if we happen to be in the wrong place at the wrong time. And in a small town like this, life for adolescents is particularly difficult. What I hope the audience will come out asking is, Who’s crazy? These kids, or their parents?” He and his producer Clive Brill consciously picked unknowns, so that audiences would not bring preconceived notions to the characters. They announced, before the auditions, that they were looking for a very large boy for the lead role. As Brill says: “We had families driving in from all over America, competing to stick their poor fat offspring on front of the camera. But we went for the best actor, not the largest Mike.”

The actor cast in the role is actually well-known in America, and not exactly unknown in Britain: 15-year-old Sean Weiss, aka Goldberg The Goalie in the Disney Mighty Ducks films. And he is sensationally good in this new and demanding role.

“As soon as I met Sean,” recalls McShane the elder, “I realised that the thing was in good hands. He’s a real comedian, a damn good actor, and he has utter confidence. Unlike my younger self, and unlike many other fat kids who get routinely picked on at school, he’s quite without paranoia. He doesn’t look over his shoulder at the world.”

Weiss, whom I encounter on the day when he has to get beaten up with a cricket stump, and who is accordingly sporting a splendid cosmetic wound, has been acting on television since he was six, and relishes the fact that he’s playing opposite his role’s originator. “It’s real cool. If you have any questions, you can just go to the actual guy. And this is a story which everyone can relate to.”

Quite so. When Brill and Bould held their auditions in New York, they questioned each hopeful on this point, and were staggered by the number who said yes: broken up with their parents, been in therapy, got into trouble with the law. And they’ve struck gold: there are faces here we shall see more of, notably a crazy little New Yorker called Rebecca Kramer, and a dynamic young hustler called Harley Cross, with the satanic charisma of a young Jack Nicholson. Cross, like Weiss, has been working since infancy, and - again like Weiss - is already rich.

The film’s mad-Anglophile psychiatrist is played by Bud Cort - whose own debut was as the suicidally-troubled teenager in Harold and Maud. When I catch him on set - instructing his charges in the mysteries of cricket - he’s been on the project for a mere 24 hours. Another actor had been cast, but had then been ignominiously drummed out for failing to evoke the requisite battiness.

“Everyone needs to do an occasional kamikaze mission, where you have no time to prepare, and have to draw on your own resources,” says Cort. “I accepted the job because it was a wonderful script, touching and strange.” Like everyone else, he’s now hoping the film will take the trail blazed by Enchanted April, with a British release following success in America.

They’re filming in Ohio, a state sometimes described as being like Norfolk, but with more interesting leaves. For mile after mile, you drive through cornfields, and past neat little frame-houses with perfectly-kept gardens. It does seem very boring.

But then you realise that these houses have large plaster ducks on their porches, all dressed appropriately for the season. And every so often you come upon something even stranger.

Would you believe, beside an unassuming little shack, a series of cages containing real-life lions and tigers? (These, by the way, have been woven into the film.) Ohio - like McShane’s Kansas - is deeply weird. After a while, you feel McShane’s question is well worth posing: who’s crazy? the kids, or their parents?

Crazy for a Kiss will be shown on BBC2 on April 23 at 10pm.