30 Dec 2001: As a Protestant appearing in a film about the still controversial events of Bloody Sunday, Cold Feet’s James Nesbitt has undertaken his most daunting professional challenge yet.

Belfast, one cold Sunday afternoon. James Nesbitt is standing alone at the crowded bar with a bottle of beer in each hand. He is wearing a stripy shirt, a blue denim jacket, blue Levi’s and Hush Puppies. The outfit looks strangely familiar, not because it is the uniform of the modern mid-thirties man, but because these are all clothes he has worn in Cold Feet. He empties the first beer into his mouth and as he is about to start on the second, everyone jumps up and cheers: Manchester United have just scored.

He looks up at the big-screen television, smiles and raises his bottle. A few minutes later, his friends appear. These are his oldest and closest friends; they grew up together in Coleraine, a small town 30 miles north of Derry. Step owns a carpet shop and Peter a suit shop, but Jimmy Nesbitt – James when he’s acting, Jimmy any other time – likes having his old mates around. It helps keep him grounded if he ever begins to be swept away by the success of Cold Feet.

We stand and watch the game. Nesbitt has been a Manchester United fan since he was a child, since he dreamt of playing alongside his hero, George Best. This isn’t a good game for Jimmy, as Manchester United eventually lose 3-1 to Arsenal. He is a serious supporter, going to as many home games as he can during the five months of the year that Cold Feet is filmed in Manchester, and by the end of the afternoon he is crestfallen.

It is early, not yet 6pm, and Nesbitt and his mates want to go to another pub. We drink Guinness in a beautiful old pub once frequented by Best. Jimmy is an easy talker, by turns excitable and emotional, funny and serious. He is also, perhaps unusually for a celebrity, a good listener. Step chats about his kids and Nesbitt announces he has another baby on the way (married for 10 years, he already has a young daughter) before they reminisce about drinking sessions. Nesbitt reminds Step of the time he drank vintage claret out of his boot. Although the story has probably been told 100 times, the laughter reduces them all to tears.

Soon it is time to eat, at a nearby Italian restaurant. Nesbitt spends most of the meal signing autographs, for young men and middle-aged women alike. He is endlessly patient, even when asked time and again to speak to blokes’ girlfriends on their mobile phones. He even laughs at this, aware that men are not threatened by him, saying it must be because he lacks film-star looks. Really it is because he is the most popular actor in Britain right now.

A little later, we go back to the Europa Hotel and watch Cold Feet on television. This is not an issue of vanity, it’s just that Nesbitt doesn’t see the finished episodes until they are broadcast. He orders a pot of Earl Grey tea and sits on the kingsize bed sucking his thumb. It is strange watching the show with Jimmy; he doesn’t seem to watch himself as much as the other actors, lavishing praise on Hermione Norris for her scenes as a woman on the verge of alcoholism and laughing at John Thomson’s ever-expanding girth.

Although he doesn’t come to Belfast to see his friends as often as he’d like, Nesbitt doesn’t want a big night, doesn’t want to stay up till dawn. He is not here to be a magnificent drinker. Tomorrow, he will be driving to Derry for the first time since he finished filming Paul Greengrass’s television film, Bloody Sunday . Tomorrow, he will be returning to the city that helped change his life.

The early-morning drive from Belfast is stunning, the late autumn trees burnt orange and gold. Eating Worcester Sauce-flavoured crisps and drinking mineral water, Nesbitt is like a small child, jumping around in his seat as he drives: ‘I’m so excited about going back to Derry, I really am.’

He is not particularly neurotic but he does worry about things, and he feels guilty about acting not being a noble calling; he is very conscious of the fact that his father is a retired headteacher and his three older sisters are all teachers. Nesbitt was going to be a teacher, too, but he managed only a year of a French course at Belfast University before moving to London to attend the Central School of Speech and Drama.

Despite the phenomenon of Cold Feet, Nesbitt found himself questioning his job, wondering if he was really achieving anything: ‘I’ve never felt that acting was my vocation, never had that tortured thing. I love acting but it doesn’t feed my soul.’ A couple of years ago, he bumped into Greengrass at the Bafta awards, and the director of The Murder of Stephen Lawrence asked if he would be interested in a film Greengrass was making about Bloody Sunday. ‘When they sent me the script I was up in Manchester filming Cold Feet. It had an extraordinary effect on me.’ Nesbitt shakes his head. ‘A few weeks later, Paul came up to see me. He didn’t want to discuss Bloody Sunday itself as much as my background. I was very honest with him right from the start.’

Nesbitt wanted to make two things clear from the beginning: he was born into a Protestant family in County Antrim, he played in flute bands with his father and until he read the film script, he knew very little about Bloody Sunday, 30 January 1972, when British paratroopers shot dead 13 unarmed Catholic men in the Bogside area of Derry during a civil-rights demonstration against internment without trial.

‘I had just turned seven that month and so I was too young actually to remember it. But even when I was 11 and my family moved to Coleraine, which is just 30 miles away from Derry, I still knew nothing about it. It was not something that was taught at a Protestant school during that period. And I suppose there was a kind of collective guilt about it too, so it was never discussed in my family or by my friends.’

Greengrass wanted Nesbitt to play Ivan Cooper, the civil-rights campaigner and one of the organisers of the march. Cooper was a Protestant but he was also an MP in Derry, where Catholics are in the majority. He felt passionately about internment without trial, which since its introduction in August 1971 had seen the imprisonment of around 900 people, virtually all of them nationalists.

Nesbitt was terrified at the idea of taking the role. He was overwhelmed at the idea of portraying a man who led the peaceful yet illegal march, who tried to keep the crowd calm by directing it away from the soldiers’ barricade in William Street. Who looked on panic-stricken as about 200 young men broke away and began throwing stones at the soldiers. Who was defenceless as the paratroopers opened fire, using lead instead of rubber bullets and killing the 13 civilians in less than 30 minutes.

As we drive across Northern Ireland, Nesbitt shudders. ‘Before I read the Bloody Sunday script, I have to admit I hadn’t though about it that much. There was probably even part of me which assumed there was no smoke without fire. That the Catholics who were shot must have done something to provoke such a response from the army. I was extremely ignorant of the whole situation.’

He says he was barely aware how it is now widely accepted that the British Army effectively damaged itself on Bloody Sunday. Until that day, the IRA had been on the verge of defeat; on the Monday morning after the massacre, gangs of embittered young men were eager to sign up.

‘When I read the script I was staggered. Astounded. I really couldn’t believe it. I certainly felt guilty about my ignorance and quite shameful too. But then again, I grew up utterly away from Northern Ireland’s hot spots. When you’re brought up in a Unionist culture, you can’t help but feel Unionist.’

Although Nesbitt had a long-term relationship with a Catholic girl until he met his English wife, Sonia, he always felt uncomfortable with any kind of religious or political extremes, particularly with nationalism. ‘While Catholics were talking about 500 years of oppression, Protestants, in turn, were being oppressed by the IRA. I could never understand why they blew up civilians in the name of a war against an occupying country; I was always very alienated from them and their cause.’

When he was offered the part of Ivan Cooper, Nesbitt read Don Mullan’s book, Eyewitness Bloody Sunday, and Peter Pringle and Philip Jacobson’s Those Are Real Bullets, Aren’t They? He went to Derry on his own to meet Ivan Cooper and spent five hours discussing Bloody Sunday. He talked to the relatives about their 30-year battle for justice. He sat in front of a Calor gas heater with them and watched television broadcasts of the ongoing Saville inquiry into the killings.

He laughs. ‘I just thought, “Bollocks, I can’t do this. I’m not a good enough actor. How the hell am I going to do justice to this, and how are the relatives of the Bloody Sunday victims going to react to me, Jimmy Nesbitt, the Protestant actor from Cold Feet, swanning into town to make a movie about their personal tragedy?” It’s important for me as an actor to be able to visualise myself as a character in a setting, and I just couldn’t place myself in Derry in 1972. I told Paul I was scared, and he spent some time just talking to me; he’s so incredibly articulate and impassioned that he gave me the strength to continue.’

Greengrass says working with Jimmy on Bloody Sunday was like watching an athlete preparing to compete in the biggest race of his life. ‘For an Irish actor, doing the Troubles is like doing Lear. My heart really went out to him when he met the families of the Bloody Sunday victims. I could see him thinking, “I’m really going to go for this.” And I think he has delivered a piece of his soul in this film. He’s got that magical thing which connects him with the public: humanity.’

Shot in black and white with hand-held cameras, Bloody Sunday is compelling, harrowing yet unsentimental. The images stay long after the film is over, images of the young lads out of control, hurling stones at the barricades, of General Ford instructing the paras to ‘Go and get them’, of the relatives in the hospital finding out that they had lost a father, a son or a brother that day.

For James Nesbitt, it is a coming-of- age film, a film about his country and for his country.

We arrive in Derry and drive past the murals of the iconic hunger striker Bobby Sands, past the graffiti – it mainly reads ‘IRA’, but also ‘England, do you really think it’s over?’ – and on to the hotel. Here we meet Cooper, now in his sixties but with the sartorial flamboyance of George Melly. He gives Jimmy a bearhug and they start chatting straight away, taking up where they left off.

It is cold and wet outside, but we drive across town to the playing field where the march began on that Sunday in 1972. Cooper jokes that at least back then it was a crisp but bright, sunny day and as people turned up in their thousands, a carnival atmosphere developed. We walk down a winding road to the Bogside.

The view of the river and mountains is striking, somehow at odds with the idea of Bloody Sunday having such a brutally urban setting.

As we approach the Catholic area, I almost expect to see blood-stained streets, perhaps because, as Derry-born novelist Seamus Deane once wrote: ‘Bogside was once a street; now it is a condition.’ Instead, the flats in Rossville Street where five young men were shot – six of those killed were aged just 17 – have been demolished. The dilapidated Victorian terraces have been replaced with respectable maisonettes but then you notice the hunger-striker murals and the ‘Welcome to Free Derry’ slogan painted proudly in large black letters on a gable end, and remember this is a city with a terrible history.

Finally we arrive at the Bloody Sunday memorial to those who were killed. Cooper talks about Bernard McGuigan, the 41-year-old friend who was shot dead in front of him. He says he has never been able to speak to McGuigan’s widow since that day, even though he often drives past her house. He doesn’t know what to say; he still carries the guilt of having insisted that the march went ahead.

It is cold and windy, and Nesbitt’s nose is red, his eyes watery. Cooper is not only affected by the elements, he is close to tears. Nesbitt stands close to him, painfully aware that this is a broken man, who has never recovered from one Sunday almost 30 years ago.

Suddenly, there is a shriek from a car window. ‘Jimmy Nesbitt’s arse! Will you look at that!’ The woman is beside herself. Nesbitt and Cooper burst out laughing, leaning into each other like old friends.

Later on, we go to a pub with a late licence. Nesbitt orders a succession of pints of Guinness and chats a little about Manchester United’s run of bad form, but mostly about Bloody Sunday. He is worried about his parents, who wanted him to take on the project but don’t really understand why their son hasn’t made a film about a Protestant tragedy. ‘It will be hard for my parents to watch, I know that. But they also know it was a terrible wrong…’

His flow is interrupted constantly by people coming up to him and hugging him, toasting his Guinness, handing him mobile phones to speak into. They are male, female, Catholic, Protestant. Most are Cold Feet fans but all know why he has spent so much of the past year in Derry. Greengrass used to joke that Nesbitt had a pint with everyone in the city while filming Bloody Sunday and perhaps he wasn’t so far from the truth.

As the evening wears on, Jimmy becomes increasingly emotional. ‘The whole process of making Bloody Sunday was difficult but extraordinary. I am still emotionally drained by it. I’m going to sound like a wanker, but it was a voyage of discovery for me.’

He tips the last of the Guinness into his mouth. His eyes are sparkling in the dim light of the pub. ‘You know, I’ve always been mouthy, always been one for the craic but I’ve never been fulfilled, I’ve always felt a bit empty somewhere. Now I can finally say I’m proud of myself.’

Bloody Sunday is on ITV1 on 27 January