It is pouring with rain, thunderclouds gathering with theatrical intensity over the Soho streets, yet when William Boyd walks into the room he is absolutely dry. Mysteriously, he does not seem to be carrying an umbrella or even wearing a raincoat, but his brown hair is fluffy at the edges and his striped scarf, tucked neatly underneath the lapels of his jacket, is pristine. There is not a hint of dampness about him.
And yet, partly because I have been re-reading Any Human Heart in preparation for our meeting, I'm not entirely surprised by Boyd's unruffled appearance. After all, this is the author who constructed a 500-page novel purporting to be the personal journals of Logan Mountstuart, a fictional writer whose 85-year life spanned each decade of the 20th century. Boyd's technique was so convincing that, at the time of its publication in 2002, many readers believed Mountstuart to be real. The novel came complete with footnotes and an index that included a reference to "William Boyd, biographer of Nat Tate". Boyd had indeed written a 1998 biography of Nat Tate, an American artist from the 1950s. It was just that Nat Tate was also a figment of Boyd's imagination (the name is a contraction of the National and Tate Galleries).
Confronted with this textual hall of mirrors, it is hard not to feel that the 58-year-old Boyd, the author of several acclaimed novels and screenplays, is capable of making us believe anything he wishes. Perhaps, I find myself thinking, the rain outside doesn't actually exist; perhaps Boyd has simply made it up.
"I like to invent things," he concedes with a smile, speaking with a soft but noticeable Scottish lilt – his parents came from Fife, although he was born in Accra, Ghana (then the Gold Coast), in 1952. "I think the well-functioning imagination, supplied with the necessary facts and figures, actually gets you very close to the truth, whatever that is." Does he believe that fiction is in some sense more truthful than the dry, hard facts of history? "If you do it well, I think you get a picture that is without the constraints of the historian or the biographer. You can push the envelope."
In Any Human Heart, Boyd's eighth novel, he set out to tell the story of the 20th century, not as a chronological catalogue of world events but rather by showing how important moments impinged on the life of an ordinary man. The result is a seamless blurring of fact and fiction – Mountstuart crosses paths with many of the century's most influential cultural and political figures, including Virginia Woolf, Ernest Hemingway, James Joyce and the Duke and Duchess of Windsor – but at the core of the book beats a fictional heart. Pivotal historical occurrences – the build-up to the second world war or the political encroachments of Mussolini in Italy – frequently merit only a distracted line or two in the middle of a description of a successful dinner party or a particularly memorable holiday.
"I think, in a way, that's part of its appeal," says Boyd, who has also written the screenplay for the Channel 4 adaptation, "because you live his life as he does. It's not that classic third-person, past-tense account that you get in novels, or particularly biographies or autobiographies, with hindsight shaping everything. The journal takes everything that life throws at you as significant: that girl met in the pub could be your next wife or someone you never see again. So you have to reproduce that sense of living life on the hoof, of living life as we all do, with the future this blank void in front of us and not guessing the significance of anything."
Still, there were certain people and events that Boyd knew he wanted his protagonist to collide with. "Yes, this is the great bliss of writing novels in that anything you're interested in can become grist to your mill. I knew he was going to meet Virginia Woolf, for example," Boyd says, laughing.
Woolf is given short shrift in the book, with Mountstuart dismissing her as a supercilious shrew. "I think I taught Virginia Woolf for too long – too many tutorials on To The Lighthouse," Boyd says by way of explanation (he was a lecturer in English Literature at St Hilda's College, Oxford, from 1980 to 1983). "But a lot of Logan's betes noires are my betes noires, whether it's Woolf or Jackson Pollock or the Duke and Duchess of Windsor or whoever. I sort of steered his life towards places or encounters or events that would allow me to explore my prejudices and my likes. It's sort of deliberate, but it seems random – he winds up in Nigeria during the civil war because I was living there then and wanted to write about it."
Unsurprisingly, Any Human Heart required a great deal of research. Boyd spent 30 months reading hundreds of books, including the complete works of William Gerhardie, a renowned 1920s author who then faded into obscurity alongside the brighter light cast by Evelyn Waugh, and who provides the key model for Mountstuart: "Gerhardie published his last book in 1940 but he died in 1977, so there were 37 years of silence, which is actually what I think is interesting."
Boyd says he was used to the idea of "thorough scholarship, but researching a novel is so different from researching a thesis because actually you're looking for something that catches your eye. You're like a magpie looking for something shiny that will serve your purpose."
The Duke and Duchess of Windsor are among Boyd's most successful characterisations. The duchess is depicted as a woman of potent sexual charisma, who speaks slightly too closely to people's faces, with the effect of making them feel they are intensely fascinating. The duke, by contrast, is skewered with the telling detail that he never says thank you. In the screen adaptation, this caused much consternation with actor Tom Hollander, who plays the duke and had to keep curtailing his natural politeness while filming. "He's constantly taking things – money, cigarettes, whatever – and never says thank you once," Boyd says. "Of course, in the novel you can point that out, but in the film it's sort of subliminal."
But one of the challenges of choosing to write a fictionalised journal is that the plot becomes necessarily shapeless and loose-ended: there is no overarching narrative thrust. Plot lines are lost and seemingly integral characters drop out of Mountstuart's life without warning. As a result, when Any Human Heart was first published eight years ago, it met with mixed reviews. Was Boyd ever worried that the lack of structure would be too confusing for the general reader?
"Yes, I was, and I think particularly because I knew it would be a long book… My worry was: could that intimate confessional tone of voice and its intense subjectivity be continued over 500-plus pages and seven decades? The answer is yes, I think, but it was a real worry initially.
"You end up having to take enormous pains to seem artless and spontaneous. So that paradox is there: all your instincts are saying: 'Tell the story, move things along', but actually you realise that life doesn't conform to the narrative of a novel, so you have to meander, there have to be passages of deep boredom and tedium, and there have to be false starts and dead ends in order to make it seem real."
Despite the reservations of some reviewers, Any Human Heart has proved enduringly popular. Boyd says he receives more letters about it than any other book he has written. But although he has met with success, he never seems to have received the same critical acclaim as, say, Ian McEwan or Martin Amis. Perhaps Boyd dislikes the thought of showing off in print. At his core, he remains a storyteller who refuses to let his lyrical style overwhelm the narrative substance of his plots. Does he feel there is a tendency among modern writers to ignore the importance of a good story?
"It depends. Fashions come and go, but I think at the root of a novel is story and character."
He is, he says "ridiculously, absurdly pleased" with how the television adaptation has worked out: "I'm almost embarrassed to say how pleased I am. I rather cheesily feel proud."
Then, with a final smile and a nod, he leaves, our hour together destined to be just one more meeting in a life full of random occurrences that might or might not turn out to be important. The rain, I notice, has cleared.
Any Human Heart is on Channel 4 at 9pm tonight and for the following three Sundays