Arts: When will the Great War end?

Scott Fitzgerald did it. So did Faulkner and Chandler. Many a novelist since has moonlighted in the movie business, but it's usually been in a position way down the food chain, as writer, whose usual fate is to see their painstakingly crafted words mangled and trampled underfoot by the behemoth of commerce. The balm for this indignity? Money, of course.

William Boyd has done his share of screenwriting too, adapting his own novels (Stars and Bars, A Good Man in Africa) and those of others, including Mario Vargas Llosa and Evelyn Waugh. Now he's gone one better and commandeered the director's chair on The Trench, a small independent movie filmed over six weeks last November.

Its subject could scarcely be more harrowing. Set over the 48 hours leading up to the Battle of the Somme in 1916, it trains a beady yet compassionate eye on a squad of young soldiers as they prepare for the big offensive on the morning of 1 July. As zero hour approaches, anticipation turns to dread at the prospect of their brief lives coming to a sudden and violent end. "I wanted to do something I felt passionate about," says Boyd, an affable presence whose voice carries the merest hint of a patrician Scots burr. "I thought we hadn't seen the real trench experience on film. All these images we see every Remembrance Day are, almost without exception, fakes - they were all shot in training camps. So I started searching for the real stuff, which is incredibly rare - all you can see in actual film of battle are tiny dots in a field. There's no cameraman up there with them because the equipment was too unwieldy."

Boyd had done his research while preparing to write his fourth novel, The New Confessions (1987), a great tragicomic scramble through the 20th century as witnessed by a maverick film director, John James Todd. The most memorable passages in the book - some of the best in modern fiction -recount the 17-year-old Todd's horrific experiences as an infantryman on the Western Front, which inspire him to make a war movie. That ambition became Boyd's own. The other prompt was seeing the director's cut of Das Boot, Wolfgang Petersen's U-boat film of 1981. "I thought it was so cinematic and claustrophobic. That's when the lightbulb went on over my head: what other experience is exactly like this? And of course it's the trenches - you can make it a microcosm. It also meant a feasible budget; you didn't need 10,000 extras or $50m. You could pack the authenticity into a few hundred yards."

Confined to this narrow strip, the film gives off the high stench of mud, putrefaction and death. Boyd was a stickler for accuracy, consulting, inter alios, the historian Martin Middlebrook, whose superb book The First Day on the Somme is the starting-point for any student of the battle. The first of July turned out to be the most catastrophic day of slaughter in the history of the British Army, with nearly 60,000 casualties - 19,000 of them dead. Within a studio, Boyd and his team recreated a trench at the southern end of the front, checking with Middlebrook about the presence there of the Royal Fusilliers - "just so we didn't fall into the pedant's trap. We tried to cover everything, from haircuts and weaponry to the actual look of the trench. I thought soldiers might have stubble, but you shaved every morning in the British Army, whether going into battle or not. Quite extraordinary; you were put on a charge if you didn't shave."

Did he feel nervous as a fledgling director? "It helped that I'd served an apprenticeship as writer. I felt prepared for all the technical detail, camera movement, composition of shots and so on. And I'd worked with really good film directors - Bruce Beresford, Jon Amiel, Pat O'Connor - friends whose advice I sought. The crew [most of them Oscar-nominated at one time or another] provided world-class back-up. I also storyboarded the entire film last summer - I drew every shot, like a strip cartoon of the movie, just so I wouldn't be stuck when asked `Where do you want the camera?'"

But the demands of rehearsing and directing his cast were rather more complicated. "I think actors lose their trust if they think you're bullshitting. The advantage in being a writer-director was that you'd invented their characters, but you could also give them room to move.

"I was talking to Paul Nicholls [who plays 17-year-old recruit Billy, the focus of the story] in the scene when the little guy brings him a mug of tea and says `Sorry about your brother', and I told him we had to have some reaction at the end when he puts his hand on Billy's knee. Paul didn't know what he was going to do, so we just said, let's see what happens. And he spat at him - which was brilliant, and his idea. That was the sort of spirit in which we did things."

What might shock audiences is the heartwrenching callowness of these boy soldiers, some of whom were as young as 16. Boyd's own grandfather was a 23-year-old sergeant who was wounded at Passchendaele, while his great-uncle was wounded at the Somme ("It's been a sort of family lore"). Yet he hasn't romanticised these young soldiers, who can be spiteful and thieving as well as courageous and sensitive. As he puts it, "There were nasty little toe-rags and nice, shy boys. It was vital to capture that, so we decided on no movie stars, and people cast to age. It was quite disturbing to see these young actors uniformed up - it just reminded you how young they actually were."

I wonder if the collaborative nature of film-making seems a less purely artistic enterprise than the solitary craft of writing. Boyd is in no doubt. "People ask me if my novels have been influenced by my film writing, but it seems to me the other way round. The generosity of the novel is in a way a perfect tutor for telling a story in film. The old line that all the devices of cinema were already present in the novel - flashbacks, parallel action, voiceover - is basically true. Look at Madame Bovary - it's all there. Film is a lesser art than the novel or painting or music, because you need this industrial process to create it. The Trench was an original script from a story I invented, but I couldn't have made it without everybody else. I don't think I'd ever be seduced away from literature, because the artistic satisfactions don't begin to measure up." His magazine on the cafe table where we're talking bears this out: it's not Variety but the Times Literary Supplement.

That said, Boyd is keen to make another film. Talking to this mild-mannered, genial, bookish fellow, you're struck by the absolute contrast to the traditional megalomaniac director figure. Did his newly acquired power encourage any latent tyrannical tendencies? "I don't think so," he smiles, "though if there were a monster in me waiting to be unleashed, then the autonomy of a director would allow it. It was a happy set, actually, though I hope the adage isn't true: happy set, bad film! The temperament of the director does determine the mood, I think. I've seen the unlikeliest English directors turn into monsters of egotism and wilfulness, but that's probably in their nature anyway. I hope I'm not that type." He pauses, and adds: "I didn't throw any chairs."

`The Trench' is released on Friday