David Threlfall: 'Celebrity? I hate the f@#$**! word!'

I have just pulled into Shameless Central. Sited on an industrial estate in Wythenshawe, Manchester, this newly constructed studio houses all the sets for the hit Channel 4 drama, which returns next week for its seventh season. Over there is the Jockey pub, complete with obscene graffiti daubed on the loo wall; and here is the back entrance to the home of the central family, the Gallaghers, adorned with all manner of fly-tipped rubbish. Outside is parked a pink stretch limo, the gleefully over-the-top ride which the Gallagher's neighbour, Micky Maguire, has pimped for his own nefarious purposes.

Even amid the encircling gloom of a December evening, the whole place reeks of the Chatsworth Estate's defining quality, its sheer joie de vivre. It lifts the spirits and reminds you of Frank Gallagher's joyous paean of praise to the "Chatsworth buccaneers" at the start of every episode: "First and foremost, the most vital necessity in this life is they know how to throw a party! Heh, heh... Scatter!"

It is this unabashed exuberance that makes Chatsworth residents such fun people to spend time with, and which has turned Shameless into one of the most iconic and original series of the last decade, with one national newspaper last week voting it among the greatest TV dramas of all time. And no one epitomises that unashamed lust for life better than Frank.

Above the Chatsworth police station – whose reception is decorated with crime-prevention posters warning "When you're out on the lash, don't flash the cash" – are the production offices for Shameless. They are decorated with no fewer than three fetching glass mosaics of the dipsomaniac Frank in his trademark pose: hair lank, eyes bleary, fag alight, flicking Vs at the viewer. It is the definitive Shameless image.

David Threlfall, the actor who has brought Frank so memorably to life over the past six years, and who has also directed several episodes of the programme, comes down the corridor to greet me. Gesturing to the mosaics, I ask if he has one hanging in pride of place over his mantelpiece at home. Threlfall looks horrified at the very idea. "It's like Bill Hicks used to say: 'When Jesus Christ comes back, he doesn't want to see figures of himself around people's necks.' I don't want to see Frank when I get home. Shit, you have to leave him at the door. You do not want him coming home with you!"

This is Threlfall all over – sharp and self-deprecating. He is, simply, everything that Frank is not. But for all that, the actor is very grateful to his dishevelled alter ego, as you might imagine. On the surface, Frank is not a character we should warm to at all; indeed, he is an egotistical workshy yob, who is addicted to benefits and booze and who would sell his grandchild for the price of a pint. And yet, in Threlfall's portrayal, Frank has become one of the most popular characters in modern television.

It is a bravura piece of acting to make someone so loathsome, so loveable. Fans can quote verbatim from his rambling pontifications, and there is a roaring trade in T-shirts quoting his most celebrated homily: "Make poverty history! Cheaper drugs now!" Regular viewers revel in the rough poetry of Frank's bar-room philosophising and the keenness of his warped wit. At one point, Frank sighs to his earnest daughter, "Debbie, love, you're as miserable as Morrissey eating Kentucky Fried Chicken at an animal-rights festival." On another occasion, when his son Lip tells him, "You're covered in sick," Frank replies quick as a flash: "Son, you are ugly. Tomorrow, I will be clean." Even his drunken meanderings are engaging. He once railed at a fish: "Now you know what I feel like. Stop whingeing! Drink more!"

As Threlfall and I sit down in a meeting room upstairs at Shameless HQ, the mosaic of Frank is still giving us the V-sign from the wall, but we carry on regardless. The 56-year-old actor gives very few interviews and is averse to blowing his own trumpet. "There have been some approaches for me to write my autobiography," he says, glumly, before adding: "Don't be silly! I'm quite shy in public."

But today Threlfall seems very much at ease. He has sloughed off Frank and is dressed in his "civvies" – a grey collarless shirt and black combat trousers. Sitting with his feet up on the sofa, the actor seems as relaxed and poised as Frank is restive and plastered.

So what has the character of Frank given him, then? "Gallstones," Threlfall deadpans. "No. Frank has made me more recognisable. People really do seem to love him. He's a licensed fool. People tune into him and say, 'I didn't think anyone else thought that.' They come up to me all the time and say, 'My cousin's just like Frank!' And I reply, 'You're proud of that, are you?' "

The resounding success of his screen creation has also endowed Threlfall with a greater sense of artistic liberty. "Paul Abbott [the creator of Shameless] has described me as 'someone who will take the ball and run with it'. Tony Garnett [the veteran TV producer] recently criticised scriptwriters for writing to order because the people above them are living in a perpetual state of fear. So it's harder all over to get things made. But the type of show that Shameless is does give you a certain freedom. It encourages you to go with your instincts as an actor and a director."

The truth is that Threlfall has long been well-regarded within the industry – he was Emmy- and Tony- nominated as long ago as 1979 for his performance as Smike in a memorable RSC production of Nicholas Nickleby – it's just that Frank made him a star. And in 2006 – 27 years after playing Smike – he finally won the Best Actor gong at the Royal Television Society Awards. Crucially, though, the acclaim has not gone to his head.

These days, Threlfall lives a tranquil life in the south-east of England with his wife, fellow actor Brana Bajic, and their two young sons. He pretty much shuns the limelight. And, suffice to say, he will not be appearing any time soon alongside the cage-fighters and ex-boyfriends of Katie Price on Celebrity Big Brother. "Celebrity? I hate the word!" snorts the actor.

"We never go out on the social scene," he continues. "I do get invited to a few movie premieres, but I never fancy them. If you go to those parties where you have to stand for hours with a plate in one hand and a glass in the other, you just come away thinking, 'God, my calves ache!' I suppose I'm a social misfit, but if it's a choice between that or a night in with the family, I'd always go for the night in with the family."

Threlfall – who grew up Manchester, where his father worked as a plumber – is mightily relieved that fame did not alight on him when he was younger. "I'm glad it's happened to me later on," he says. "If it had happened to me earlier, I'm sure I'd have done some stupid stuff – don't even ask me what but I'd probably have screwed up! Now I have spent enough time with myself to understand how I – and the business – operate."

When he's not acting, Threlfall spends much of his time editing or preparing to direct his next episode. So, does he ever relax, I wonder? "It's hard being away from the kids," he replies, "I think they actually like me as well as love me. So when I'm back home, I just see the family. I do normal things like walk the kids to school in the morning and pick them up in the afternoon. I'm pretty dull!"

Threlfall trained at Manchester Polytechnic School of Drama before making his professional debut – opposite Ray Winstone – in Alan Clarke's 1977 film Scum. He takes his work very seriously indeed. The director Trevor Nunn once called him "a handful" and Threlfall confesses that, "I frighten directors a bit because I have to try everything out before settling on how I'm going to do it. So, in rehearsal, I might be Rasputin one day and Mother Teresa the next!"

He thinks, though, that his outlook has mellowed somewhat with the passing years. "I'm still as intense as I was as a younger man. But with time comes a certain relaxed approach. Some people [on Shameless] have gone through various difficult things. I say to them, 'This is only a play – go and sort it out.' You have to have balance about what goes on in the world."

Which is one reason why Threlfall has signed up to help with the campaign to free Tibet and to be an ambassador for Save the Children. "Save the Children's work is focused on children and health – pretty fundamental stuff. Children are our future, as the song says. When you have children yourself, it arouses this primal thing within you. And now, because this show has made me recognisable, I can do something about it. I'm really pleased to be involved in a low-key way... he said, as he went into print about it!"

Threlfall is the first to acknowledge that the path of his career has not always run so smoothly, however. "I went through a bad patch a few years ago. For a couple of years, I couldn't give it away. I was within a day of becoming a motorbike dispatch rider when Jude Kelly rang with the offer of Odysseus Thump at West Yorkshire Playhouse."

Today, though, he is viewed as a small-screen titan. His Shameless co-star Pauline McLynn – who plays Frank's new love interest, Libby, in the latest series – describes him as "brilliant, perhaps the top actor in the country".

He certainly boasts a certain chameleon quality, equally able to play characters from opposite ends of the social spectrum – from the Duke of Drunks (Frank Gallagher in Shameless) to the Duke of Edinburgh (Prince Philip in The Queen's Sister).

But despite this versatility – and popularity – Threlfall says he's not taking anything for granted. "When Shameless stops, I hope the people who give out jobs won't just focus on Frank," he says. "There's a 20-odd-year back catalogue before that. I'm always striving to do different things. You always want the other thing, the thing that you're not doing at that moment. The grass is not necessarily greener on the other side – it's just different. It's more of a concrete patio!

"I'm well aware that fashions come and go," he continues. "What's popular can change just like that. Longevity is the thing to aim for. The secret is to keep your head down, wear a tin hat and avoid the bullets."

The new series of 'Shameless' starts at 9pm next Tuesday on Channel 4