Morpurgo does not pretend to be a horseman himself. She must stand there – because that is what friends do.' The horse wasn't understanding, but was listening. I summoned his teachers and we hid in the vegetable garden to watch them. He was with Hebe, a gentle Haflinger mare: 'And he was talking to the horse, one to one, about what he had done on the farm that day. Then, one rainy evening, he spotted the boy outside wearing slippers.

During his stay, Morpurgo observed how 'adept he was with farm work'.

His teachers explained that he was severely withdrawn, with a stutter. In 1978, a boy from Birmingham visited the Morpurgos' Devon farm as a result of their charity, Farms for City Children. And he goes on to tell me the wonderful story without which novel, play and film would never have materialised. He says: 'I have been on quite a ride,' as he settles down in Bafta's bar in Piccadilly, London. While waiting for him to put down his schooling whip and get in touch, I catch up with Michael Morpurgo and congratulate him on War Horse's amazing trajectory (the novel has topped the New York Times bestseller list). Horse trainers are famously elusive (and/or hopelessly tied up with their horses), and Bobby Lovgren, Spielberg's head horseman, is no exception. And even if you are not horse-struck, it is impossible to watch the film – with almost 200 horses in its cast – without wondering: how did these creatures of flight face combat? And how, in the film, were they trained to 'act' with such dazzling compliance? More than a million horses were sent to the first world war for use by British and Commonwealth troops only 62,000 came back.

It is the horses that are the film's star performers, with their gleaming flanks, flowing manes and hooves thundering through history. The film has a first-rate British cast – Emily Watson, Tom Hiddleston, Benedict Cumberbatch and newcomer Jeremy Irvine. He had what neither book nor stage could boast: real horses. And that must be, at least partly, because Spielberg knew, right from the start, that he held a trump card. Early signs are that War Horse the movie is conquering America and is likely to vanquish British audiences as well. Now, in Steven Spielberg's hands, the story has become epic. And that might have been glory enough – except that War Horse was also, always, a film waiting to happen. The story became today's Black Beauty, a sentimental education, a must-read classic, a global hit. Marianne Elliott and Tom Morris's National Theatre production – still stabled at the West End – entranced audiences with its uncanny, life-sized horse puppets. The book was short, accomplished and moving, but barely acknowledged until, in 2007, it was turned into a play. It started in 1982, with Michael Morpurgo's novel about a boy called Albert and his horse, Joey, who is sent to fight on the bloody battlefields of France in the first world war. W ar Horse has had an extraordinary career.

The war horse michael morpurgo