How would it be if instead of our natural life span being around 80 years or so, it was 30 at most? That's what happens to the characters in Mark Romanek's Never Let Me Go, a film adaptation of the Kazuo Ishiguro novel which, although set in rural England in the 1990s, exists in an alternative reality where humanity has learned to clone itself and schoolchildren are conditioned to accept that in their 20s their lives will end for the greater good.

"I wanted to create this world in which young people become old while staying young," says Ishiguro. "For some peculiar reason their life spans are concertina-ed. I was concerned about asking what really becomes important when they realise their time is short.

"If there's somebody dear to them that they feel they've treated badly, will they want to put things right before it's too late? If they've loved somebody all their life but never had it acknowledged, will they want it acknowledged and want to have some time together even if it's only a short time? I try in my book to be positive about human nature so the things that become really important are things like friendship and love."

Never Let Me Go, which stars Keira Knightley, Andrew Garfield, Carey Mulligan and Domhnall Gleeson, is a metaphor for what everyone experiences sooner or later in actual life. Talking with Ishiguro about mortality reminds me of a playwright friend who in his last years tried to contact anyone he might have offended and apologise: as it happened not all of them would accept his remorse.

"Was it John B Keane?" Ishiguro asks.

"No, Hugh Leonard."

"I just asked that because when I was in Listowel, the town of Keane, I thought he'd be a celebrity but it seemed he was at war with nearly everybody. I wondered what happened when he died."

"He was well-remembered."

Ishiguro and I hadn't met since I interviewed him in 1987. "I still have the cutting," he says. He'd yet to write his Booker Prize-winning novel The Remains of the Day then, but had published two novels set in Japan, where he was born. He was five when his family moved to England from Nagasaki in 1960. As a leading oceanographer, his father was involved in research on North Sea oil. Every time they were almost about to return to Japan the project was extended.

Ishiguro's schooling was thoroughly English, and English became his natural idiom: he could hardly write or read a word in Japanese. There wasn't any talk at home in London's Golders Green about the bomb that ravaged Nagasaki, although his grandfather died from the effects of radiation after helping to pile and burn the corpses. As a child he just thought every city had a bomb. When he grew up he avoided going back to Japan, afraid that being there would disturb the impression of it he was constructing in his fiction.

"I eventually went back a couple of years after I met you. It was remarkably similar to the way I remembered it as a small child. The neighbours were the same. The corner shop lady was the same. I realised that for a long time a tiny pocket I remembered on the outskirts of Nagasaki was the entire country for me. I mistook it for Japan. Tokyo could have been a different planet."

He didn't originally intend to be a writer. He made demo tapes and busked in the Paris metro before a radio play, written while working with the homeless in Glasgow – where he also met his Scottish wife – gained him a place on the creative writing course run by Malcolm Bradbury and Angela Carter at the University of East Anglia. Almost immediately Faber and Faber accepted his first short story and gave him an advance on a novel. "My dream always had been to be a songwriter. I've now gone back to writing songs for the American jazz singer Stacey Kent."

He would like his novels to take on a life of their own as songs do. "If you're a songwriter, what you want is that a lot of people will adapt your song and it becomes a standard and people do their own versions and it enters the cultural stream. For me it's deeply flattering if a novel I write doesn't just exist as a novel but that it goes further and they make movies or plays out of it or even, in the case of The Remains of the Day, they make a musical out of it. That to me is a source of real satisfaction, not whether people have been incredibly faithful to the book."

Never Let Me Go, which was shortlisted for a Booker Prize but lost out to John Banville's The Sea, was particularly difficult to write. "I tried to write it twice in the 1990s and each time I abandoned it and wrote a different novel. I couldn't find a convincing way of creating a situation in which the way of life of the characters would just seem normal for them. Then I tried again in 2001. I imagined what would have happened after the second world war if the breakthrough in science hadn't been in nuclear physics but in biotechnology, and then wondered where we might have got to by the late 1990s."

His story is about people rather than cloning. "I wasn't interested particularly in the infrastructure of this world, or how it was supervised and enforced. All I had in my head was this parallel reality. I'm interested in what people say to themselves about themselves, and the way they skirt over uncomfortable things and try to put a positive spin on things they are unhappy about. That's why I always write in the first person. I think the actors in the film capture it awfully well. They're particularly good at non-verbal acting. Over and over again they come out with a line and there's a tension between the words that come out of their mouth and what their face and body are actually saying."

All the time Ishiguro was writing Never Let Me Go he was discussing ideas with his close friend and neighbour Alex Garland. "He was writing his sci-fi movies 28 Days Later and Sunshine. We've been meeting in local cafes and restaurants for a long, long time, talking about the ideas in our heads. So it felt very natural when he said, 'Look, I want to have a go doing the screenplay.'

"I tried to keep a back seat during the filming. I've always worried about filmmakers being over reverential to the book because I have worked as a screenwriter too. I wanted them to feel that they should only put something in the film if they really felt it for themselves and they weren't paying lip service to something else."

Ishiguro wrote the screenplays for the Merchant Ivory film The White Countess and Canadian director Guy Madden's The Saddest Music in the World. "I'm always thinking about original film projects. I've another in mind right now for Guy Madden. My life wouldn't be the same if there wasn't some kind of film possibility going on alongside the novels."

'Never Let Me Go' opens on 11 February

Who's that boy?

When Keira Knightley and Carey Mulligan face imminent death in Never Let Me Go, they turn to Domhnall Gleeson for support. "It's only a small part, but he's explosive," says Kazuo Ishiguro. In True Grit, it's the other way round for the 27-year-old Dublin actor: he's an outlaw looking to bounty hunter Jeff Bridges for mercy. "A great guy to act with," says Bridges. True Grit opens the Berlin Festival next week. Gleeson, who won a Tony nomination at 23 for his Broadway performance in The Lieutenant of Inishmore, will later this year figure in the 2011 Shooting Stars showcase as one of Europe's 10 most talented newcomers. Meanwhile, dad Brendan Gleeson isn't doing too badly, with rave reviews for his rebel cop in John Michael McDonagh's The Guard, which opened the Sundance Festival.

Ciaran Carty