“The past has always obsessed me,” says John Banville. I look carefully at him to see if this is a Banvillian joke: a moment earlier, as we made our way to the bar of the Merrion Hotel, he had been riffing with considerable glee on the vagueness and vagaries of the 66-year-old memory. “It’s absolutely true,” he protests. “The middle stuff is all gone. But the past is incredibly vivid. We think we’re living in the present, but we’re really living in the past.

“That’s one of the themes of the novel; that we live most vividly in the past. It’s a strange thing. Why does the past seem so magical, so fraught, so luminous? At the time it was just, ugh, another boring bloody day. But, to look back on, it’s a day full of miracles and light and extraordinary events. Why is this? What process do we apply to the past, to give it this vividness? I don’t know. And that’s why I keep probing at the problem.”

The past certainly has a starring role in Banville’s new novel. Ancient Light, his 19th since the story collection Long Lankin was published, in 1970, finds the elderly actor Alexander Cleave writing a memoir about his illicit affair, as a young teenager, with the mother of his best friend, Billy Gray.

As Cleave beavers away in his attic room, his wife is suffering from night terrors that cause her to wander the house searching for their daughter Cass, who died by suicide in mysterious circumstances. Is Cleave’s retreat into a sunny long-ago summer an attempt to escape from this overwhelming grief? If it is, it’s doomed to fail, for when he is invited to play the role of an enigmatic literary critic named Axel Vander in a Hollywood movie, the layers of Cleave’s reality get darker, weirder and more uncertain than ever.

Regular readers of Banville will already have spotted some familiar points of reference. Cleave made his first appearance in the novel Eclipse (2000), when he fled to his parents” house to mourn his dead daughter and his ailing acting career. Shroud (2002) took up the story from Cass’s side. A researcher who has discovered dastardly deeds in the past of one Axel Vander, she goes to Italy to meet Vander, and ends up throwing herself on to the rocks at a Ligurian resort.

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Although technically part of a trilogy with Eclipse and Shroud, Ancient Light stands as a novel in its own, majestic right. It is, it almost goes without saying, ravishingly written and scrupulously observed. But it’s also a highly playful book, full of puns and puzzles and hilarious – or are they vicious? – pastiches. There’s an academic, Prof Blank of the University of Arcady. At one point the narrator conjures up a doll from childhood: a 3D, horror-movie moment.

In his efforts to get into the role of Vander, meanwhile, Cleave is reading a biography that the film company has supplied. Called The Invention of the Past, it’s written by “JB”, whose style, Cleave notes with some asperity, is “wholly unnatural, synthetic and clotted”.

Later in the book Cleave meets JB for a drink but wishes he hadn’t. “JB is distinctly odd, and grows odder each time I encounter him,” he declares. “He maintains a furtive, anxious air, and gives the impression always of being in the process of edging nervously away.”

Not just JB but also, to a T, John Banville as he likes to think of himself, rather than the urbane, amusing sophisticate who turns up to interviews and literary conferences.

JB – sorry, Banville – picks up his wine glass and looks gloomily into it. “People don’t see this in my books, often,” he says. “I never intend them to be funny. You can’t set out to be funny; the jokes would fall as flat as lead balloons. But the humour arises from situations. I think Mrs Gray is funny,” he adds. “Well, she finds him screamingly funny. He’s a boy, you know? And teenage boys are ridiculous.”

And, in their rampant and confused desires, vulnerable. Does he worry that readers might find the book’s full-on depiction of a 15-year-old boy with a 35-year-old woman shocking? “Well, you see, I never thought of any of this,” he says. “It seemed to me perfectly straightforward – and I hope there won’t be a controversy, with people coming for my blood.

“It seems to me a perfectly innocent relationship. I know that might seem strange. But here are these two people who need comforting, and they comfort each other in the only way they can. Because they can’t really talk. There’s nothing for them to talk about. Mrs Gray just waffles on, and Alex has, as she says, one thing on his mind. But this is what human beings do, offer each other comfort as best we can. We take it where we can find it in this terrible world. And why wouldn’t we? So I couldn’t find anything shocking in these two lost souls. Well, one is a lost soul. One is a soul that hasn’t found itself yet.”

Even innocence isn’t quite innocent in Banville’s portrayal of the relationship, with its glancing but persistent references to incest and religiosity and its almost forensic examination of the complexity of any, never mind family, human interactions.

In this respect the text is, its author insists, self-determining. So, sometimes, are the characters. The book’s evocative title is supplied by an Argentinian called Fedrigo Sorrán, who materialises in a hotel bar and talks to Cleave of “heat death and the Hubble constant, of quarks and quirks and multiple infinities”. The ancient light is the light of galaxies, which takes as long to reach us as our universe has existed.

It’s a reminder that Banville is the author of an earlier trilogy – Dr Copernicus, Kepler and The Newton Letter – about scientists. “I”ve always been fascinated by physics and cosmology,” he says. “It gets more and more scary the older you get. Since I’ll soon be out there.”

There is, however, a very unscientific explanation for the mysterious Mr Sorran. “He’s a Spanish friend of mine called Rodrigo Fresan – of course it’s an anagram of his name – and he asked me. He said, ‘Give me a walk-on part.” So I gave him a walk-on part. But he’s an interesting character, isn’t he? Because I didn’t know where he came from, or who he was, or why he was talking about ... being a miner, wasn’t it?

“I used to try to control everything I did in my books. Now I let things happen, and frequently that’s where the best stuff comes from. And people will tell me all sorts of things. There’s a book coming out later in the year, a Banville reader. The editor tells me that Cotter’s Place, the abandoned house where the lovers meet in Ancient Light, is in Birchwood. There’s a Cotter’s Place in Birchwood where the boy sees his parents making love, or something like that. It’s obviously deep in my memory, but I have no memory of it at all. Stuff accumulates in the mind like lint, and it just floats up without you noticing, which I like.”

It’s a mark of confidence in his ability as a writer – isn’t it? – that he’s able to allow such material to find its place in his beautifully balanced prose.

“Confidence or increasing senility,” he says. “One or the other. I suppose it is. I feel that I don’t have to worry so much about the technical aspect of the language. I”m at last beginning to learn how to write, after all these years, and I can let the writing mind dream.”

A major element in Banville’s literary progress, though he doesn’t take kindly to the suggestion, has been his decision to write, under the name Benjamin Black, a series of crime novels featuring a delightfully mismatched pair of investigators, Det. Insp. Hackett and his pathologist friend Quirke. Since Christine Falls appeared, in 2006, he has produced five Black books and two Banville novels, The Infinities and Ancient Light. Having spent a couple of days reading first the latter, then the most recent of the crime novels, I was surprised by the similarities I found.

“Oh, God.” Banville looks appalled. “You’re not supposed to say that.”

I”m not saying the books are alike; not in the slightest. But they share preoccupations, even images. Happening, in Vengeance, upon the sentence, “The leather seat was hot where the sun had been shining on it”, it’s hard not to recall the paragraph in Ancient Light when Alex and Mrs Gray “climbed back into the front seat, exclaiming at the hotness of the leather where the sun had been shining on it ... ”

Banville nods grimly. “Did Benjamin borrow his ...? Yeah. They’re both completely ruthless. They will steal from each other mercilessly; they have no shame. Of course they leak into each other. They have to. I can’t be two people. Banville will get interested in a sentence that Black is doing, or Black will say to Banville, ‘Just write the bloody thing. Leave it alone. Stop worrying about it.” ”

Has writing crime novels changed the way Banville plots the literary novels, or modified his storytelling? “I don’t think so,” he says. “But I suppose everything we do influences everything we do. And I suppose Black is getting more subtle as the books go on. He’s getting more interested in the characters. Apart from anything else, he’s had them for a while. I mean, I”m writing the sixth book now, and I”m interested in Quirke’s daughter Phoebe. I think Phoebe is me.” Really? Not Quirke himself? “No. I don’t know Quirke at all. Curiously, when I think about Quirke and when I think about Benjamin Black, I mix them up. I think of them as the same person. But they’re not me.”

And what is it about Phoebe that is John Banville? “Well, she’s so troubled. So fascinated by life’s oddities. And she doesn’t quite fit into things. There’s more of me in her, certainly, than there is in Quirke.

“And you know” – his eyes take on an impish, dangerous twinkle – “Quirke is a big man in his 40s, impossibly attractive to women and so on. When I”m doing readings I see readers looking at me and going, ‘Oh. He’s very old. And he’s very short.” So I say to them, ‘I”m not Quirke. I”m not even Benjamin Black.” But I have a good time writing the Black books.”

The Quirke books are due to be made into a television mini-series next year, with Gabriel Byrne as Quirke. The Man Booker-winning The Sea is also to be filmed, with Rufus Sewell and Ciaran Hinds in leading roles and Banville at the screenwriting helm. He’s working on an adaptation of Turgenev’s A Month in the Country. Failing memory be damned. John Banville’s writing mind is clearly dreaming at the speed of light.