David Hayman was sitting in a pub the other week, watching the Celtic vs Rangers match with his sons and some pals.

After Rangers went one nil up, there was a shout from the back of the room: "Haw, Lear, whit ye gonnae do about that?" Quick as a flash, Hayman replied: "I'll have them all beheaded on Monday." The entire room erupted into applause.

"That's Glasgow for you," smiles the actor and director, who was born in the city. Although perhaps best known from his 12-year tenure as Detective Mike Walker in Lynda La Plante's cops and robbers TV show, Trial And Retribution, he looks at his most comfortable in Glasgow's Citizens Theatre. Sitting in the foyer in a black woolly hat and checked shirt on a soup-and-sandwich break from rehearsing the title role in Dominic Hill's new production of King Lear, it's clear Hayman has come home to roost.

It was in this building, after all, that he became the enfant terrible of a remarkable company formed at the dawn of the glorious 34-year reign of artistic directors Giles Havergal, Robert David MacDonald and Philip Prowse.

It was Hayman, now 64, who caused a scandal from the off playing the title role of Hamlet in a production that made front-page news with its scenes of near-nudity and outrageous acting styles that inspired the headline: This gibbering idiot will surely close the Citizens. Another decried the show as a "naked Hamlet".

"The schools freaked out and cancelled their block bookings," Hayman recalls. "But all the kids decided they really wanted to see a naked Hamlet, and they queued around the block every night. It was fantastic."

Forty years on, and with Hayman something of an elder statesman of stage and screen now, whether Lear can match such stuff of legend remains to be seen. However, one thing is certain: Hayman's relish for the challenge of one of theatre's mightiest roles.

"It is a masterpiece," he says. "It's one of the greatest plays in the English language. It's so rich and so dense, and it raises so many issues about life, social justice, greed, the abuse of power, capitalism, the bonds of family. It's all in there."

While open and generous to all inquiries, Hayman won't be drawn on his interpretation of Lear.

"I'm still in a process of exploration," he says, "but of course there are parameters. He's a very tough leader, who makes a foolish decision, falls from grace, gets rejected, ends up penniless and without his support systems, and then goes through a period of madness. Out of this madness comes a clarity and lucidity as he sees humanity for the first time through a pauper's eyes, and he appreciates the man he's not been, but who he is beginning to be. He's a king who learns to be a man."

If such a well bullet-pointed dramatic arc doesn't quite match Hayman's personal and professional development during the decade he spent at the Citz, his time there remains a crucial rite of passage.

"This was my creative home for 10 years," he says, "and it was an extraordinary 10 years. I always wanted to be part of an ensemble company. I believed in theatre, and never wanted fame. I didn't come into the industry for that. I was perfectly happy to be in the Gorbals exercising my craft. I had a unique opportunity. Between the ages of 20 and 30 I played Hamlet twice, Lady Macbeth, Troilus, Nijinsky and Petrucchio. That's a wonderful training ground for any actor that's not available these days, and hasn't been available for a generation."

Hayman left the Citz in 1979, then came back to direct John Byrne's Slab Boys trilogy for Mayfest. "The last time I appeared on the stage was 22 years ago," he remembers, "and when I came here before Christmas I just stood on there on that stage, and got goose bumps.

"For six nights a week, 10 months a year for 10 years, I was on that stage, and rehearsing during the day. That's an extraordinary commitment to a space, a theatre and a company. I'm sure lots of reverberations will come back when I'm on that stage again and in front of a Glaswegian audience. It's real here. There's something visceral."

Hayman may be a key figure of Citizens history, but it wasn't a place he'd set his sights on after graduating from RSAMD (now the Royal Conservatoire of Scotland). His immediate port of call was the Greek islands, where he bummed around, contemplating whether he even wanted to be an actor. It was only on his return to Glasgow that he found out that Havergal, who'd watched his final diploma scenes, was looking for him.

Hayman became the core of a company which took full outrageous advantage of the social and artistic freedoms the 1960s had bequeathed society.

"There was a creative angle of trying to look at the classics afresh, but Giles, David and Philip always believed that theatre should be scandalous. They weren't political, but they questioned the sexual mores of the time. That's why there was a lot of cross-gender casting."

This was a long way from Hayman's beginnings as an apprentice in a steelyard in Possil after being kicked out of school at 16. One day, the boiler-suited ingénue found himself walking off the bus to work from his Drumchapel home and up the steps of RSAMD, where he heard his voice tell the receptionist that he wanted to be an actor.

"I don't know where it came from," Hayman admits. "I was crippled with shyness as a teenager, but I think it might have come from going to see pantomime when I was a boy, with all that cross-dressing and gender confusion going on there as well."

However, it took until he played convicted killer Jimmy Boyle in John McKenzie's big-screen version of Boyle's autobiography, A Sense of Freedom, for Hayman's father to warm to the idea of his son being a successful, now mainstream, actor. By that time Hayman had already departed the Citz.

"It broke my heart to leave, but I guess we had to move on. I wanted to explore film and television, which I did, first as an actor, then as a director. That was a period I really enjoyed, and I didn't miss acting at all."

While his stage directing continued, he cast a young Iain Glen as Boyle's Barlinnie contemporary in his 1990 feature, Silent Scream, then followed a path that led eventually to Trial and Retribution.

Hayman returned to the stage in 2007 for Six and a Tanner, a one-man play originally penned by Rony Bridges for Oran Mor's A Play, A Pie and A Pint seasons of lunch-time theatre. Following a tour that included dates in HMP Barlinnie, the production looks set for an Edinburgh Festival Fringe revival. Last year Hayman appeared alongside Jude Law and Ruth Wilson in Eugene O'Neill's Ann Christie at the Donmar in London's West End.

"There's a lot of my industry I'm very disenamoured with. Like the world, television is run on greed and ego. It's not creative integrity, and that wears thin after a while. Prancing around London in an Armani suit in front of a camera is never going to change the world. It's pure entertainment, which is fine, but I've never wanted to be an entertainer or a celebrity. I'm a performing artist, so rekindling my love of theatre has been wonderful."

If Hayman sounds totally energised talking about acting on-stage, he moves up yet another gear when he mentions Spirit Aid, the charity he set up in 2001 to initiate humanitarian projects in areas of the world ravaged by war, poverty and other man-made ills.

"It's the most important part of my life," Hayman enthuses. "The world is going to hell on a banana skin, and I couldn't just sit back and do nothing. Our moral compass has gone off beam. We spend more than a thousand billion dollars on arms, and 1% of that money could feed and educate everyone in the world."

With an office just across the Clyde, Hayman is again getting back to his roots. He quotes a piece of native American philosophy that could easily be something Lear learned the hard way.

"'We do not inherit the earth'," says Hayman. "'We borrow it from our children'."

King Lear, Citizens Theatre, Glasgow, April 20-May 12. www.citz.co.uk