In the wings of the London film festival, a group of young influencers are discussing their personal highlights. Klara and the Sun, says one, and her colleagues agree. It’s not just that the film is still so fresh in their memory, or that they have gathered today to meet its creators and stars. Klara is funny and moving; it’s their festival favourite so far. I relay this exchange to Taika Waititi, the film’s director, who is sipping bottled water in a room down the hall. His gaze widens. He almost chokes. “Thank God!” he exclaims with just a bit too much heat.

Everybody has a perfect film, just as everybody has a perfect friend, and they vary because everybody is different. In the film, sick 14-year-old Josie loves her discontinued robot doll Klara, whereas most kids her age opt for brighter, shinier models. And the young influencers love Klara and the Sun – Waititi’s adaptation of Kazuo Ishiguro’s acclaimed novel – while most older critics vehemently, violently disagree. All of which shows that there’s no accounting for taste, that people are confounding, and that our hearts and our brains are not always in sync. That keeps life interesting. It’s partly what the film is about. “The emotions that we give very simple labels to are really complex,” says Ishiguro, who is seated alongside the director. “The thing we call love, for example, is incredibly noisy and multifaceted. Nobody knows what the hell love is.”

For what it’s worth, I loved Ishiguro’s distinctively spare and poignant source novel and am less convinced by Waititi’s screen version, which is certainly noisy. Jenna Ortega stars as Klara, the perky solar-powered AI who plays soulmate to adolescent Josie (Mia Tharia), at a remote country house. She’s a warm splash of sunshine in an otherwise frosty environment, a fish out of water in a tragicomic sci-fi caper. But there is a glitch in the matrix; the component parts don’t align. Waititi adds sugar and caffeine, pratfalls and one-liners. Klara misreads social cues and steers the family car off the road. She fears she’ll be shipped back to the factory and melted down for scrap.

Perhaps the perfect friend is independent, self-directed. Ishiguro has seen several of his Nobel prize-winning books – The Remains of the Day, Never Let Me Go, A Pale View of Hills – adapted for the screen. He feels that the best films break away and that the worst are too faithful. “Sometimes it’s not just loyalty but laziness,” he says. “You can’t just have the text there in front of you. You need to use your imagination to move it on. A film needs to step forward, out of my book on a journey to being something bigger.”

Waititi agrees. “Why would you do a cover of a song with exactly the same instrumentation? You make the film because you love the book. But you at least want to bring some original ideas to the table.”

The book and the film are so different, it follows that their creators would be too. Ishiguro is sober, soft-spoken and writes delicate dramas about the deep mystery of life. Waititi is voluble, peacockish and makes uproarious films such as Jojo Rabbit, What We Do in the Shadows and Hunt for the Wilderpeople. He tends to view the world through a comic lens. “Yeah, that’s my instinct. I can’t make a Schindler’s List, that’s not in my wheelhouse. I want to push everything towards comedy and then reel it back.”

It’s a defence mechanism, he admits, part of his cultural DNA. “I’m from New Zealand,” he says. “In New Zealand, my friends and I always had a very sensitive cheese-o-meter. We thought the Americans loved cheese and saw sentimentality as a good thing. But of course that’s also me being scared of it. Now I think there’s a way of being more emotional and dramatic. Hopefully that’s a sign of me maturing.”

Waititi’s work, though, has always carried a pleasing eddy of sadness. His 2010 feature Boy was a comedy about child abuse, while What We Do in the Shadows reframed the vampire as a lost, lonesome immigrant. Klara and the Sun is a love story but it’s also about isolation: the gap between people; the limits of understanding. “Oh, I’m definitely interested in loneliness,” he says.

Ishiguro chips in. “Yeah, I think you’re right, there’s a crossover between our preoccupations,” he says. “But this isn’t just everyday loneliness, it’s something more fundamental. It’s about the human experience. There is part of each of us that remains alone. There is stuff that we all have to face alone. Not least death.”

That’s the other big looming topic of Waititi’s film: death. Klara works as an “artificial friend” while other robots serve as “bereavement dolls”, filling in for lost spouses, siblings and children. That’s a sound business model, the director insists. “As a father with two kids, I always fear I’ll lose one. The younger me would have thought, ‘Oh, you can’t replace someone special.’ But the older I get, and the more in love with my kids I get, I think, ‘Oh, eventually I’d believe it was them.’ The human brain is easily adaptable. A therapist will tell you that you can programme anyone to fall in love. Tell someone you love them 50 times a day and in a few months your brain will give up and believe it. So I’d definitely consider replacing one of my children.” He laughs. “They don’t even have to be dead. I’d just like a version that’s nice to me and doesn’t ask for money.”

Ishiguro once joked that he essentially writes the same story in a variety of guises. He is drawn to flawed eyewitnesses and a partial view of events; groping narrators who try to make sense of their places in the world. The Remains of the Day’s rigid butler doesn’t know he’s in love, nor that the stately home where he works contains Nazis. Never Let Me Go’s hothoused schoolgirl finds out she is held captive and being used in terrible ways. Ishiguro’s books are unique but each connects to the world, and sometimes in ways he could not have foreseen. Klara and the Sun, for instance, was recently cited as a potential handbook for the development of generative AI. Tom Lue, VP of global affairs at Google Deepmind, suggested it be used as an “ethical touchstone”, the centrepiece of a “human-centred” approach to the future of frontier AI.

So the book has been co-opted. It has taken on another fresh life of its own. Waititi turns to the author. “It’s all your fault,” he says. “That’s incredible,” says Ishiguro. It’s the first he’s heard of it. “I know a lot of people within the AI industry are worried about its impact on society. It’s got to the point where they feel the need for a handbrake. So they are surprisingly keen to talk to creative people and road-test various ideas and scenarios.” He frowns. “But here I do become protective of both the book and the film. Because our story really isn’t about all that. If someone is trying to figure out the ethics of AI and where we should all be going, I’m not sure they’d be well-guided by this story. There are other, better places to go.”

Waititi pipes up. “The Terminator’s a good one.”

Klara has been built with a sweet, simple purpose: to provide friendship and support. But the human world is hazardous, unstable, which means she has to update her files and adapt her approach. Waititi’s film veers and darkens, riffing off its downbeat source novel, finally closing in towards death. “There’s another version of this story where Klara would turn into a murder-bot,” he points out.

Robot dolls fill a need, plug a gap and ideally make life better. But artificial friends, by their nature, are replaceable too. In the press room next door, the influencers are still comparing notes, killing time between interviews. Klara and the Sun is receding; the conversation has already moved on. One says: “You have to see La Bola Negra. It’s my film of the year.”

• Klara and the Sun is in cinemas from 22 October in Australia, and 23 October in the UK and US