'Were you out last night my friend?' 'Is it that obvious?' I say pulling down a wrinkled lower eyelid to expose an eye as bloodshot as those of the two men opposite me. They are hot young actor Michael Fassbender - all fair brown hair, blue eyes, slight frame and Kilkenny accent, and veteran Dublin thespian Liam Cunningham - older, beefier but kinder looking. We are sitting in a windowless antechamber in one of San Sebastian's grandest hotels. There's a glass table between us upon which Cunningham is flicking the ash from his cigarette in the absence of an ashtray. Fassbender is nursing a large and lurid Bloody Mary.

We are here to talk about Hunger, film debut of Turner Prize winning artist Steve McQueen about the emotive events that led to the 1981 hunger strike at the Maze Prison and the death of provisional Irish Republican Army (IRA) member and local MP Bobby Sands (played by Fassbender). The film is showing at the seaside city's annual film festival and the boys have come to enjoy some late summer sun and shoot the breeze about an important film that they both seem to know may just be their legacy.

It's before noon and a clearly tired Fassbender irritably fields all too familiar questions about the weight loss regime he followed in the film. 'Yeah, I had to get down to 58 kilos. At 58 your body mass and fat ratio are stable, any lower than that and you get into the danger zone. It took ten weeks and yes I did find the whole process humbling. When you deprive yourself to that extent, of course you are going to start appreciating the smaller things in life.'

'It's a remarkable thing, a real achievement what he did,' murmurs Cunningham with a sleepy smirk. Fassbender's uniquely entertaining turns in 300, Angel and Wedding Belles suddenly seem a long way away. Is this what they call the martyr complex? I decide to keep it simple and ask how he became involved in the film.

'The film's casting director Gary Davy gave me the script. I looked at it and thought "Jesus, what the fuck?" The first act was pretty sparse in dialogue and then you get to act two and it's like this fucking avalanche of words, it's like a cascade, and I'm thinking, "How do you tackle this?" And in the third act it's starvation and death. I was thinking "Who the fuck wants to watch this for an hour and a half?" So I thought "I don't think I want to do this.' Fassbender takes another hit from the hangover fixer and visibly seems to mellow. 'So - God bless him - Gary called me up and was like "Before you make your decision you've got to meet this guy, Steve McQueen" and I thought "Yeah of course I'll meet him. How stupid of me, and then once I got in the room there was this large black man from England wanting to make a film about Bobby Sands, and I was like, "Why the fuck do you wanna make a film about Bobby Sands man?" And he was like "well 1981 was the year in my life that I remember. The Brixton riots kicked off, Tottenham won the FA cup and there was this guy called Bobby Sands on the news every day who had such a powerful effect on society and the government and I wanted to make a film about it." So I was like "OK, and I want to make a film with you." There's such a humanity about Steve that makes you want to work with him'

Written by Enda Disco Pigs Walsh and McQueen, Hunger is a lyrical, detailed and brutal portrait of prison life and recent British history and an all too prescient lesson in how not to deal with freedom fighters or terrorists (depending on which side of the sectarian or otherwise divide that you stand). The film is, however, crucially underpinned by a long single take in which Sands meets Cunningham's Father Dominic Moran in the empty prison canteen. After initial humourous banter, Sands reveals that he is going to lead a new hunger strike to protest for the reintroduction of special category status for Republican prisoners. The conversation turns into a battle of words in which Moran tries to change Sands' mind. It's an unforgettable scene, at its mention Cunningham places his cigarette upright on the table and gently sweeps the discarded ashes in to a small hillock. His moment to proselytise has come. 'The script was so good. I mean it's beautiful writing, it's a very delicate and a fantastic scene written by someone who wants to ask very tough and difficult to reply to questions.' Years of treading the boards and finding his motivation have given Cunningham a winning candour. He continues. 'From a technical viewpoint I thought it was going to be a nightmare anyway, but when I arrived to Belfast, Steve said: "I'm thinking of shooting the scene in one shot." I said "Are you out of your fucking mind?" And he said, "No, it will be good."

Fassbender gives Cunningham a sideways glance and butts in, all the time fiddling with the bright green straws in his now empty glass. 'It's such a muscular layered piece of writing that we knew we couldn't muck about with it too much - you start breaking it up and you lose the rhythm, you lose the texture and all the different sort of colours in that piece, you know?'

Without missing a beat Cunningham continues his acting masterclass: 'Yeah, so because I hadn't met Michael before, and because we only had five days to rehearse the scene, we had to move in with each other. Every morning Michael would get up and make porridge and then we would run over the scene again and again to just break it down and get it into our bones. Steve was shooting during the day. He would come in the evening and give us notes and we would discuss certain pieces. When it came to shoot the scene, we were ready.'

For a first time director, McQueen has certainly managed to install a loyalty in his experienced cast, warmed by his friend's considered loquaciousness Fassbender adds, 'You know when you're working with an artist … I don't know, its just different. They talk to you differently, it's a different concept of how they're making a film. When I'm working with Steve its all encompassing, it's all the senses, smell, taste, touch, each sense is equally important to him - you can see that in the working process. Also, the notes that he gives you are just sort of very textured notes, simple notes, coarse notes. It was a real pleasure working with him, and I just feel happy that it happened once in my career.'

Cunningham quickly fills the silence left by Fassbender's demonstrations, 'By contrast I was in The Wind That Shakes the Barley and Ken [Loach] kind of throws you in the deep end. He gives you historical pointers but you don't get a fucking script so that makes you work really hard to get inside the character. With Steve it's just two human beings talking in a room. To a certain extent the priest is a device to explore Sands' commitment to what he is about to do. It's extraordinary and intelligent. The priest can't understand why he would put his life in the hands of those who despise him. Sands believes, as he says, that he is the last line of defence. The way Sands has set up the hunger strike - where people take his place when he dies - he is basically signing his own death warrant, and that is suicide. As two Catholic men, that's not an easy topic of conversation. The priest is on his side but saying "Don't fucking do this, I completely disagree." What it shows is courage of conviction.'

Hunger is a stunning film, one that easily slots into the roll call of great prison-set dramas like Papillion, Silent Scream, Riot in Cell Block H and Cool Hand Luke, but is also tempered by a lyricism most evident in the works of artist/directors Jean Cocteau, Salvador Dali, Julian Schnabel and even Warhol. Like all good stories from the not so distant but more brutalised past, it shines a light on these times.

'What you have to remember' - Cunningham offers as a closing salvo as the pair rise from the table strewn with the debris of hangover recovery - 'is that at the time, outside the prisons there was no support for this. Obviously the British and Irish governments didn't want it to start, but neither did the IRA or Sinn Fein. Ultimately, however, many people saw what Margaret Thatcher did as murder and said: 'These folk just want their fucking clothes, and free association, they're not asking for much." But she was like "I ain't gonna give it to ye." If you force someone into a position where they have to kill themselves to destroy you, that's an enemy you are not going to beat. So you have to ask one question - "Why do you hate me so much that you would do that to me?"'

Hunger is on selected release from Fri 31 Oct. See review