Martin Clunes: he pass the acid test?

It didn't take long. The actor Martin Clunes has just sat down in the lounge of the posh central London hotel where our interview is being held when a man at the next table leans across and, looking impossibly pleased with himself, says: "I hope you're going to behave yourself." The Men Behaving Badly tag sticks to the actor like a particularly persistent burr on a woolly jumper. "By and large, people only see me as Mr Men Behaving Badly," Clunes sighs. "I could play Saint Francis of Assisi and the papers would still call it 'Monks Behaving Badly'.

"People come up to me in the street and refer to my character quite frequently. I was once sarcastic to someone who said something about behaving badly. I replied, 'Did you work that out by yourself, or did you have to have some help?' and he looked so crumpled that I immediately thought, 'Why have I done that?' Now when it happens, I just smile and say 'Good one.'"

These days, Clunes seems pretty even-keeled about the apparently unbreakable association with BBC 1's hugely popular lad-com. After all, he reckons the series has done him a lot more good than harm.

"If I hadn't appeared in it, I wouldn't have had all these great opportunities. Controllers are interested in audience figures, so they offer you interesting jobs to see if the "Men Behaving Badly Effect" will rub off on them. It would be churlish if I minded it; I owe it a lot. And I certainly haven't been penned in by it. I've had a fantastic variety of roles since."

Clunes has shown that there is much more to him than playing a slob capable only of lounging, drinking six-packs and fantasising about Kylie. In the past year, he has proved himself a pretty impressive character actor by taking on roles as diverse as Moliere's scheming Tartuffe at the National Theatre and the saintly Mr Chips. Now he is set to distance himself even further from über-lad Gary by playing a character devoid of laughs – the notorious Forties "acid bath murderer" John George Haigh, in ITV's drama A is for Acid.

Leaning back in his armchair, Clunes is a charismatic presence. A tall, expansive man, he is capable of being funny without even trying. At one point, he starts to stuff my pockets full of the hotel's complimentary sweeties for my children before telling me sternly not to send him their dental bills.

Dressed in beige chinos and a crisp, blue checked shirt, Clunes laughs off suggestions that A is for Acid is part of some overarching masterplan to reinvent himself as a serious actor. "Loads of people won't give a toss that this is a change of tack," he snorts. "Also, I'd go mad if I constantly tried to second-guess what people think of me. If you get too analytical, it stops you doing anything at all."

So what background work did he do for the role of Haigh, a monstrous man who dissolved his victims in a vat of acid to avoid detection? "Obviously, I've killed a lot to research the part," he deadpans, "otherwise they wouldn't let me do it.

"Further than that," he continues, "Haigh was a terrific charmer and conman, and I've employed certain tricks and charm that I've found successful on other people. All his victims considered him a friend – he should have carried a government health warning."

A pitch-black streak of knowing humour runs through A is for Acid. Haigh, a Brylcreemed, pencil-moustachioed Ronald Coleman-lookalike positively twinkles as he utters his doom-laden catchphrase, "why don't you come to my workshop?" However, we are assuredly not in "when did you last see your trousers?" sitcom territory.

"There is something strangely ironic about someone being so charming and helpful to people he subsequently kills," the actor says, "but you couldn't call it a comic situation. We won't be getting a spin-off series of At Home with the Haighs."

If Clunes's career has changed course recently, then so has his personal life. Happily married to the producer Phillipa Braithwaite, Clunes admits that his attitude to life was transformed by the birth of his daughter Emily a couple of years ago.

"Dressing up and folding other actors into an oil-drum as John Haigh are fun things to do," he says, "but being a parent puts them into perspective. There is an added pressure to keep working because those little shoes cost a lot, don't they? I've also given up smoking because I want to live forever and meet Emily's children."

Unable to keep a straight face for long, he adds: "I now think people drive their cars too fast, I watch gardening programmes, and what happened to all the tunes? The other day, I nearly put some sandals on over a pair of socks, and then I suddenly thought, 'hold on, you're not a BBC cameraman'."

But becoming a father has also made Clunes anxious to shield his family from the more intrusive press coverage. "I don't care when there's a headline saying 'Estate Agents Behaving Badly' and the picture editor has just seen the last two words and dialled up a picture of me.

"But," he carries on, "I do mind it when newspapers send long-lens snoopers to get secret pictures of me and Emily, and then soon after a journalist writes a piece about what a bad father I am for parading my child in front of the papers. I'll throw rocks at that particular journalist if I ever see her."

In addition to directing a drama about dogs in Victorian Britain and starring in a new drama about single parents, Clunes is due to appear in a revival of Men Behaving Badly on BBC 1 next year. But why, with the actor now in his forties, bring the series back now?

"There might have been a time when it wasn't a good idea," he says, "but this time it seemed like one. It might be interesting to see how the characters have developed. The writer Simon Nye has had four children, so I'll be interested to see what spin he puts on parenthood." No doubt the new series will also prompt a fresh rash of "amusing" comments from fans in the street.

Clunes is aware of the transient nature of celebrity. "It's bound to end. I've had more luck than most, so I can't get too depressed. But you have to prepare yourself mentally for obscurity and obsolescence."

Perhaps the actor has achieved so much success because he was never absolutely desperate for it. Clunes became famous by virtue of not being too fussed about fame. "I won't let it get to me. It would be exhausting if I spent all day minding about that rubbish and fuming about not being taken seriously as an artiste. But what about my theatre work, you may ask?

"It's my job, but it's only part of my life. When you're shooting every day, that does consume your time. But when I'm out walking the dog in Dorset, it all seems a long way away. I resolutely refuse to change my life just because I'm on telly."

He adds, poker-faced, "Of course, I carry a gun, but who doesn't? Guns are the new brown. Like any actor, I'm needy. But I don't see it as a quest. Some actors do, and seem to have a very important job, which I don't know about. We don't heal anything. Don't we just dress up for other people's amusement? Don't we have the same job as Coco the Clown?"

'A is for Acid', ITV1, Monday at 9pm