Peter Mullan: the swot who lost the plot

Peter Mullan's childhood has been trudged over in interviews ever since he starred in Ken Loach's My Name Is Joe in 1998, when he was already in his late 30s. He grew up in Glasgow, one of eight children in a poor family that lived under cosh of "a raping, bullying, alcoholic" father. Academically gifted, he dropped out of school at 14 to knock about with a knife-carrying gang. "I was a total tourist," he says. Although already a committed Marxist, he was aggressively lobotomising himself – while keeping up his reading on the sly. "You couldnae tell the gang you were reading Carl Jung." After a year or so, booted out of the gang, he went back to school, blitzed his highers and started at Glasgow University at 17. His dad died of lung cancer on his first day.

You can picture an earnest drama being made of all this: a sort of Glaswegian Billy Elliot, with socialism instead of ballet. That would be one way of doing it. Another might be Neds, Mullan's searing third film as writer/director, and possibly his best. Set in Glasgow in the early 1970s, it follows bright kid John (played by Gregg Forest, then as an older boy by Conor McCarron) whose aspirations take a kicking. Everyone expects John to follow his big brother into a gang; in the end, he gives up trying to prove them wrong.

How much is based on Mullan's life? He borrows a line from Dennis Potter, saying Neds is "personal but not autobiographical". These days, Mullan sports a bushy beard worthy of Karl Marx himself – grown for his latest acting role, in "Stevie's" (as in Spielberg's) adaptation of Michael Morpurgo's War Horse. Trim as you like, he is a picture of rugged health: add a pair of wellies and he wouldn't look out of place hauling in a day's catch on Skye. He is all but unrecognisable from Neds, in which he plays John's violent, drunk dad.

Mullan has played a hardened drinker before – a recovering alcoholic who lapses catastrophically in My Name Is Joe. But this is something else. Addled by booze, disappointment and heavens knows what else, his character has barely a flicker of humanity: his sole purpose seems to be terrorising his family. It's an intense performance. Is he playing his own dad?

"In a weird way, yes, it is him. And in another way, it's not. I used his words, but I put them in the mouth of a character I made up. There is not one word he says that my father didn't say. And not one thing he does that my father didn't do." He corrects himself: actually, no, he never beat his dad with a frying pan, as John does, "as much as he would have deserved it, and as much as I would have liked to. I beat him up in every way – but never with a frying pan."

He smiles wryly. In 2001, he told the Observer how, at the age of 14, he tried to kill his dad. "I completely lost it," he said. "I made him a cup of tea and I filled it with all these sleeping pills. And I went in – I'd never made him a cup of tea – and I gave him it, and he just looked up and smiled. Obviously, he didnae drink it. If my son ever thought of trying to kill me, I don't know how I could cope with it, the fact that I'd alienated him to that degree."

Other than knowing he'd wear a suit, as his lab technician father did, Mullan didn't give a second's thought as to how he should play him – until his first scene. "I panicked," he says. "But we did it in two takes." He now believes his dad was mentally ill, possibly as a result of fighting in India during the second world war. "Jesus, I've been drunk a billion times, I would never dream of doing such things. It wasn't just the alcohol: if drink does that to someone, I'm sorry, there's something else cooking up there and it ain't nice."

Mullan has four children, ranging from two and 20. Only the eldest, his daughter, is old enough to watch Neds. "She's really hard on anything I do," says Mullan, who finds it difficult to talk about his childhood to his kids, especially his middle two, aged nine and 13. "Do you say grandad was a real piece of work? Because they're asking me with complete childlike empathy, 'Was grandpa not nice to you?' I'll be honest, I've had to temper it recently. Because it was scaring them."

Too tough for Riff-Raff

Mullan is a magnificent talker; after graduating, he stayed on at Glasgow to teach. And he's a hilarious storyteller: the tales unspool with lubricated ease, as if he's sitting in a bar, pint in hand. Each comes with an army of impressions: his kids, his actors, the woman from the Film Council who visited the Neds set and thought the house, with its upstairs, downstairs, kitchen and living room, looked a bit fancy. "'My God!'" he says, mimicking the woman. "'That's a half-million pound house!' She was thinking London prices."

Like the previous movies Mullan has directed, Orphans and The Magdalene Sisters (which won the 2002 Golden Lion in Venice), Neds can be tough one minute, brutally funny the next; tender, then take off into surreal lunacy. He writes in libraries in Glasgow, where he lives, and people chat away to him. Back in his 20s, he got involved with community theatre and worked with prisoners. He was also performing with radical theatre companies and was gutted when, down to the last four, he was turned down for the lead in Loach's 1991 film Riff Raff (it went to Robert Carlyle). They said he was too old. But, as Mullan says himself, he has got a tough-looking face.

It was tough-looking when he was a 17-year-old undergraduate, the lone working-class kid in tutorials, wondering why no one ever challenged him in English lit seminars. "It's not like I was intellectually endowed," he says. "I'm not Chomsky." Later, when he was on the teaching staff, he asked a lecturer. "And this woman was looking at me as if I was insane. And she went, 'We were all fucking terrified of you.'"

Ironically, he had been trying to come off as middle-class. Mullan does a demonstration, crossing his legs, hands on knees, head tilted. "I was very sympathetic. I was heavily involved in sexual politics. I'd read Germaine cover to cover. I went through my Sylvia phase. I was working really hard at all this. And they all thought I was a ned and would stab them if they disagreed with me." Was he never tempted to play up the working-class hero? "No, I went the other direction. I was terribly over-affected."

When he was in a gang, he spent his days loitering outside the local convent school waiting for the posh girls to come out for break. "We were like a pop band in our Crombies and bovver boots." Then the nuns got wind and called the police. The characters in Neds are amalgams of kids he knew then. "Sadly few of them are still alive." What was the worst they got up to? "Nothing as bad as in the film," he says, although they did carry weapons, and he saw boys get their throats cut. "I never did any of that shit. I was always quite clever. I could get involved and yet not be involved."

After a year or so, it was over and he went back to school, where the teachers didn't trust him as far as they could throw him. "Quite rightly." How hard did he work to get into university? "Big time. I had to. I had seen the devil." When he started writing Neds, he wanted to look at knife culture, but found, as he went along, that it became something "about the darkness of adolescence". The result is a far less lovable film than anything Shane Meadows has ever cooked up.

Which brings us to what exactly a "ned" is. It's a word that's alien to pretty much anyone outside Scotland. Neds aren't tartan cousins of chavs – for one thing, they've been around for decades. Mullan reckons the most credible explanation for the origins of the word is that it's a variant of the suited-and-booted teddy boys: English teds became Scottish neds. But he also likes the acronym that has ended up being part of the film's title sequence: non-educated delinquents. "It appealed, because here's a story about a boy who is obviously educated, or wants to be. All the forces conspire to prevent him."

What you looking at, Jesus?

Those forces – and here's where Neds gets surreal – include the heavenly variety. In one blackly funny scene, John, who has been sniffing glue, has a dust-up (or "square-go") with none other than Jesus, who drops down from a crucifix outside the parish church. What was going on there? Mullan cracks a grin: "I remember the day I killed God. It scared the fuck out of me because where do I go now?"

Killed him how? By reading? "Yeah, there and there." He taps his head and heart. "I just decided he doesn't exist. From very early, I wanted to do a big comedy fuck-off punch-up when he literally kills Jesus. Because after that, where are you going to go? You can only go back to humanity. Grasp the wrongs you've done and try and get through it."

When Neds was shown at a Spanish film festival, a concerned woman in the audience asked Mullan if he was worried he'd committed blasphemy. No, he replied, it's a joke. "Because social realism really gets on my tits sometimes. You have to just go with your imagination, where your instinct takes you. I find the world more absurd now than I did when I was a kid."