James Nesbitt sips a pint in a cosy bar near his home in Herne Hill and reveals there was a time when embarrassment over playing pretend for a living, and guilt over his sudden fame and wealth, almost destroyed him.

"It's hard to remember, but there was hysteria about Cold Feet," says the 41-year-old who went from jobbing thesp to instant recognition as the charming, easygoing Adam Williams in the hugely popular TV series in 1997, then gravitas and leading man glory in Bloody Sunday and Murphy's Law, the gritty cop show that returns with a dark and shocking three-hour special that starts on Sunday.

"When you suddenly become successful, the change is enormous, both financially and in terms of recognition and the way people treat you. I found that hard to deal with. I got very guilty about it, and I think I put up obstacles to prevent myself enjoying it," he continues in a Ballymena drawl that wraps itself round you like a rough, warm blanket.

"I am a fairly happy-go-lucky person, but I think maybe I forgot that for a while. I felt displaced. Looking back - and this sounds ridiculous, because I should have been incredibly grateful - I can see that I probably wasn't very happy." He's still uncomfortable talking about all this and gives me one of those arched-monobrow grins. "As I told Piers Morgan," he says wryly, "Catholics have confession, whereas Northern Irish Protestants only have interviews."

One of the reasons Nesbitt is wary of sounding too sorry for himself is that those "obstacles" consisted primarily of a string of affairs and dalliances, allegedly beginning with a legal secretary and a Cold Feet co-star, Kimberley Joseph, reportedly escalating with a former Miss Ireland, Amanda Brunker, and definitely culminating in a drunken 2004 "kiss and cuddle" with a 17-year-old prostitute in a Dartford hotel while he was on-location filming Murphy's Law. The girl stole his money and his green Paul Smith boxer shorts.

His wife Sonia Forbes-Adam, whom he'd met on a 1989 tour of Hamlet shortly after they both left drama school and with whom he has two young daughters, stood by him, but only if he took a good hard look at himself ("my wife is a very strong woman"). Today, Nesbitt talks about the affairs themselves only in oblique terms, although he's frank about the ramifications.

"It's the sort of thing, I'm sure, that could destroy relationships," he says. "My way of dealing with it was to try and be honest about it, which meant talking to my family more honestly than perhaps we had done before. We didn't need to love each other any more, but maybe it left us better able to communicate our love for each other."

His dalliances, he insists, were an aberration brought on by the pressure of fame, not indicative of a dissatisfaction with his marriage. "I love my wife," he says firmly. "I don't think that anything that ever happened was to do with my relationship with my wife. If I had ever thought about my wife, nothing would ever have happened. Fortunately we were prepared to work at it at the time - it's a cliché, but that's what you do - and we are lucky that we still like each other, which I think is as important as love in a marriage."

If his relationship with his wife is all the stronger, though, Nesbitt is newly aware of the upset he caused his schoolteacher father and ex-civil servant mother. They took a lot of stick from the Protestant community when he played Ivan Cooper, a sympathetic Protestant among Catholics in Paul Greengrass's Bloody Sunday, he says, but his tabloid shame "unquestionably" hurt them more. His parents are uppermost in his mind partly because he, Sonia, eightyearold Peggy and four-year-old Mary have just been to see them in their seaside retirement cottage in Castlerock.

What's more, the new episodes of Murphy's Law see the undercover copper, so recklessly confident when infiltrating Loyalist criminal gangs and investigating Muslim money-laundering, utterly at a loss when faced with a mother stricken with Alzheimer's and unable to recognise him, and a father unable to cope.

I tell Nesbitt I found the scenes of Murphy with his parents very moving and he nods. "There's something about seeing a strong man not being able to cope with a sick parent," he says.

"What resonated for me is that there are some things you cannot put right. I don't know a single person who doesn't regret the things that they did to hurt their parents, or the things they didn't say to them.

"And turning 40 unquestionably makes you question mortality, your own and your children's, and your thoughts do turn to your parents more. Not just when they are going to die, but what your relationship with them has been."

He's trying to make sure he's doing right by them now, therefore, and also that he communicates well with his own girls, "although I'm pretty sure at 16 they'll tell me to f*** off. As they should."

Clearly, the long, hard look Nesbitt had to take at his marriage and his behaviour in hotel rooms, coupled to his turning 40, has made him more ref lective and introspective. He says he "wouldn't admit to being in therapy even if it were the case", but admits he's perhaps come to terms with the fact that he wasn't always the easygoing, nonchalantly confident feller-me-lad he always thought he was.

His utterly spoiled childhood as the youngest brother of three older sisters in Broughshane, County Antrim, was, he has always maintained, "idyllic". The family were "God-loving, not Godfearing", the Troubles were 30 miles away, and the kids always had Catholic friends.

When Nesbitt decided not to go into teaching like his sisters, and threw in his French degree at Belfast for drama school in London, he already had an Equity card and a certain cocky charm under his belt.

"But before I came to London I hadn't seen an avocado or eaten a tomato," he says. "On my first day at my drama school, Central, two girls in my year started talking about their periods and I was absolutely horrified. I thought I was this terribly advanced person but actually I had been closeted."

Perhaps he was similarly unprepared when Cold-footed fame came calling in 1997. "Well, before I was successful, I certainly thought acting was a very noble profession," he smiles, "but once I got successful I became ... not guilty, but embarrassed."

Now, he says he's got perspective on his career, as well as his life. "It's easy to say that Bloody Sunday was worthwhile because it addressed a higher issue, but Cold Feet was worthwhile as well because it entertained people," he says.

He won't dismiss the series that made him a household name, nor will he deny that he thought some of it was "weak".

He worried, too, he says, that the second series of Murphy's Law featured too many different undercover roles - "there was an element of Mr Benn to it" - and says he and the writers and directors have subsequently worked to darken the tone, to make Murphy's cutting humour a more obvious defence mechanism, to change Nesbitt's own "fluid" body language to something harder.

He's enjoyed getting more serious roles and taking them more seriously now he's older, even if he doesn't like feeling older. What he relishes, he says, is good scripts and the fact the comparative under-funding of film and television in the UK and Ireland means that there is no place for egos or hidden agendas. He's even stopped worrying about his hair loss since his agent told him it would never stop him getting work.

"What I've realised is that my job is my professional and natural home," says James Nesbitt, "and I have to say I'm very happy." He's drunk two pints and smoked two cigarettes during the course of our interview (he's going to stop smoking, he insists, before his next TV project, a modern adaptation of the Jekyll and Hyde story, because eight-year-old Peggy hates the smell).

Just before he gets up to go, he says: "Oh, I know what I meant to tell you: I've just been made a Unicef ambassador." Last year, he got involved in Wave, a charity that helps Catholic and Protestant children orphaned or traumatised by the Troubles, which had lost interim funding as a result of the peace process ("It's like saying: 'There's peace now - you're cured' to kids who've had their parents shot in front of them").

This led to a Unicef trip to look at Aids programmes in Zambia, after which he wrote a moving (and notably wellcrafted) article. Next year he hopes to go to Sierra Leone or Uganda to highlight the plight of child soldiers.

"It's a privilege," says Nesbitt of his ambassadorial role, "and maybe, subconsciously, it takes away some of that embarrassment, that I have about the whole acting thing. It balances it up somehow. And my mum always wanted me to be a missionary as a kid, so finally I am going to Africa and doing a bit of missionary work in my own way."

As he heads back to his own wife and kids, I can't help reflecting that, although Catholics have confession, maybe Northern Irish Protestant actors sometimes get absolution.

Murphy's Law is on BBC1 on Sunday 27 August, Monday 28 and Tuesday 29.