There is something very right about the room where I meet author William Boyd – a rather functional box at an office in Soho, seemingly stripped back for redecoration and the aluminium Venetian blinds drawn, it could be a passenger lounge at some far-flung airport, we both agree. "Dubai," suggests Boyd, several of whose novels – A Good Man in Africa, for example, or Brazzaville Beach – have been set in distant, sweatier climes.
We're here to discuss his latest adaptation of one his novels (he only allows himself to turn his own books into screenplays), Any Human Heart, the sprawling saga that follows writer and journalist Logan Mountstuart from boarding school in the 1920s to his exiled dotage in 1990s France, by way of encounters with such real historical figures as Ernest Hemingway, Ian Fleming and the Duke and Duchess of Windsor. Written in the form of a journal, it is the story of a life and a series of snapshots of the 20th century as experienced by one quixotic man of letters.
Despite sharing some autobiographical traits with his creator – such as a fondness for the game of golf, and a poor opinion of Jackson Pollock and Virginia Woolf – Mountstuart is an amalgamation of a generation of British writers with whom Boyd has long been fascinated. "Lawrence Durrell, Henry Green, Cyril Connolly..." he says. "And the one who everybody has never heard of, William Gerhardie, who was the most famous young novelist of the 1920s, the Zadie Smith of his day, and who Evelyn Waugh, Anthony Powell and Graham Greene all confessed to being hugely influenced by.
"Logan's career rather echoes Gerhardie's – he had huge success in his twenties with his first two novels and the rest of his life was a long slide into oblivion and poverty. He published his last novel in 1940 and died in 1977 – so 37 years of silence. But all these writers conformed to Cyril Connolly's theory of "Enemies of Promise" (Connolly's 1938 treatise on the obstacles to literary output)... journalism, marriage, children, drunkenness... I met Lawrence Durrell at the end of his life – he was a terrible of old soak. Logan is a lazy writer, so in that sense he's like Connolly. If he can think of a reason for not writing he will."
In that case, Logan couldn't be less like William Boyd, who not only drinks in moderation – often the produce of his own vineyard near Bergerac in France – but has also composed a steady stream of novels since his debut, A Good Man in Africa, won the 1981 Whitbread First Novel Award. And then there are the 14 of Boyd's screenplays that have been filmed and the many others that haven't – including adaptations of his own An Ice-Cream War, The Blue Afternoon and Brazzaville Beach. "I was never paid for them so the screenplays all belong to me," he says in his soft-spoken, Gordonstoun-Scottish accent.
Of his books that have been filmed, Stars and Bars (1988) starred Daniel Day-Lewis as an art expert at large in the Deep South of America, the Bruce Beresford-directed A Good Man in Africa (1994) had Sean Connery in the title role, and Armadillo was made by the BBC in 2001. Any Human Heart was one of his novels, however, that he considered un-filmable (The New Confessions is another") because of its episodic nature and sprawling length. Channel 4 and Carnival Films (makers of Downton Abbey) changed his mind when they promised him six hours in which to tell the story. And they have delivered on that promise, with Sam Claflin (The Pillars of the Earth) playing Mountstuart in his student days, Matthew Macfadyen then taking over for his middle age, and Jim Broadbent portraying him in later life. Having three actors to play the same character gets over the need for tricksy make-up, Boyd pointing to Chaplin, the 1992 biopic of Charlie Chaplin (which film's screenplay he co-wrote), as the effect he was trying to avoid.
"We had a 25-year-old Robert Downey Jr trying to play an 80-year-old man under 15 pounds of plastic prosthetics... you end up looking at the make-up," he says. "Even when you throw money at it, like in that film The Curious Case of Benjamin Button, there's something odd about Brad Pitt being 10 or being 85, so we thought – everybody knows it's a movie, they're all actors – everybody knows that – let's just say we're going to use three actors."
Claflin, Macfadyen and Broadbent are part of a well-judged cast that also includes Freddie Fox (son of Edward Fox) as Mountstuart's lifelong friend Peter Scabius, Hayley Atwell as his great love, Freya Deverell, Kim Cattrall, Julian Rhind-Tutt and (uncanny as the Duke and Duchess of Windsor) Tom Hollander and Gillian Anderson. From what I have seen – the opening episode and a showreel of the remaining three – Any Human Heart is going to be a treat. What it resolutely won't be is the novel of the same name.
"I can bore for England on this subject," says Boyd. "I keep saying this, but it's worth saying again, the novel and film are two entirely different art forms... as different as opera and theatre. I approached Any Human in the way that I would approach the adaptation of any novel – what would work on screen? – rather than saying 'let's be as faithful as we can to the book'. It's all about film-making and not about reverential attention to the source. So in a way you park your novel. "
And of course Boyd is lucky in that he didn't have the author's feelings to take into consideration. "As the author writing the screenplay you can actually be far more ruthless and audacious," he says. And the first thing that fans of the book will notice is that Mountstuart's schooldays have been completely excised, as has his later sojourn in Africa.
"All adaptations lose probably 40 to 60 per cent of the book," he says. "Sometimes these decisions make the drama better, the way a novel can meander and pause and digress just doesn't work in film. I'm always amazed at how capacious and generous is the novel form – you can do anything. As soon as you get into film you just run up against parameters, constraints and compromises, which are not forced upon you by venal producers or anything like that.
"Because I've directed a film (The Trench), and I've co-produced a film (A Good Man in Africa), I know the film business and television business right through from commission to press screening, so I sort of know what's involved. Lots of novelists think, 'I wouldn't mind having a go at that', but actually it's quite a demanding world."
In fact, unlike most novelists, Boyd has dedicated much of his working life to the art of the screenplay. "I've always loved film," he says. "I first went to Hollywood in the 1980s. That's like ancient history... the films I wrote – not all of them were always made – wouldn't even get near the front door now. I used to go to Los Angeles two or three times a years, but all my film work since then has been in Europe."
Boyd is however currently writing an American screenplay ("a sort of heterosexual Brokeback Mountain") just one of several projects he has on the go. "Some of them have been on the go for 20 years," he adds. "The longest gestating project I've ever had is this film I wrote called The Galapagos Affair, the true story of which is a murder mystery set in the Galapagos Islands in the 1930s and we've had it cast and crewed three times and it's fallen apart. The producer keeps sending me emails "imminent green light". I actually optioned the book in 1985, we once had Faye Dunaway in it which shows how long it's been on the go."
Fans of his novels will be glad to learn that Boyd has just started a new one ("I'll finish next year sometime and it will be out in 2012"). Does he think that writing screenplays has affected the way he crafts novels? Does he make them more "filmic" than before? "No. If you did that you'd write a 60-page novel. Anyway, if you're any sort of serious novelist why would you want to do that – why would you want to hogtie your world of total freedom with the limitations of cinema? Writing a novel is like swimming in the sea, making a film is like swimming in the bath.
"The quote I always use is Vladimir Nabokov, who said that films should be a vivacious variant of the novel, and I think he's right." And I think that neatly describes Channel 4's upcoming Any Human Heart – it's a vivacious variant on Boyd's novel. And what's more, as I can vouch as someone who's also currently reading Any Human Heart, it won't spoil your enjoyment of the book.
'Any Human Heart' starts on Sunday at 9pm on Channel 4