Ask any writer where they got the idea for a particular book and it’s possible they may not know. They might even look slightly puzzled, and then offer the explanation that it was just something that they always wanted to do. There is no such uncertainty from Michael Morpurgo, the author and former children’s laureate, when he considers the origins of War Horse, which is being dramatised at the National Theatre in London next month.

They can be found down in the Torridge Valley, near to where Ted Hughes wrote, farmed and fished for the last 40 years of his life and rhapsodised about “the deep lanes of Devon”. Despite being flagged up as Tarka Country by the county’s tourist board because of Henry Williamson’s classic otter story, it is a quiet, bypassed landscape midway between the county’s north coast and the fringes of Dartmoor. When Morpurgo and his wife, Clair, came to live here 30 years ago with their children, it was even more bucolic, with a community whose memories easily encompassed the Great War.

At the local pub, the Duke of York, Morpurgo met a man who had served with the Devon Yeomanry Horse Regiment and could recall in poignant detail the business of taking a horse to war. There had already been what you might call a classic teachest moment, in which Morpurgo came across some pictures left to Clair by her father, Allen Lane, the founder of Penguin Books. They had been painted by F.W. Read, and published in a magazine. “They were dreadful scenes,” says Morpurgo.

“They showed cavalry horses charging up towards some trees where German soldiers were hiding. There was barbed wire in front of the trees, and I could see that one of the horses had become tangled, really horribly tangled, with the wire.”

As you will know if you or your children have read War Horse, which was published in 1982, it tells the story of a horse called Joey who is sold to the cavalry and shipped over to France, leaving its young owner, Albert, inconsolable. Joey has a remarkable war, serving on both sides of the conflict and then finding himself alone in no man’s land. Although Albert is too young to join up, he is so concerned for Joey’s welfare that he sets off regardless to find his horse and bring him safely home.

There are further reasons why Morpurgo was drawn to such a theme. He and his wife were both teachers, both disaffected with conventional schooling. “I was aware of not getting through to about half the children in my classes,” he says. “The ones from homes where there were books, talking, walks and activities were fertile soil. But for the ones who watched TV and did little else, five hours of teaching a day had little effect.” They had come to Devon with the aim of setting up a scheme to give town children first-hand experience of the countryside. This they duly did, working with local farmers to host groups of children and teachers for week-long stays. Farms for City Children, as the programme was called, attracted such demand that some 50,000 children have now taken part, either here or at one of the charity’s two other farms in Gloucestershire and Wales. It’s a story in its own right, but its immediate relevance is that teachers and children alike were not prepared for the benefits that the animals themselves could bring to young lives.

Morpurgo expresses this by recounting one incident. “There was a busload of children, about 35 of them, from a school in Birmingham. When the teacher got off, he took me to one side and said: ‘There’s a slight problem with Billy.’ Billy had a terrible stammer and was terrified of opening his mouth for fear of being teased. Now, some kids were scared of the big animals, but not Billy. I saw him with a horse, when he thought no one was looking, and he was talking to him and his stammer had gone. The reason that Billy clammed up was a fear of being judged. There was a tenderness between the two. Animals, for their part, seem to understand the absence of threat from a child.”

In the context of all this, it did not seem absurd to have Joey tell the story, nor to think himself into the character of his equine narrator. For the production at the National, scripted by the playwright Nick Stafford and directed by Tom Morris and Marianne Elliott, the horse will also be the starring figure, played by a life-size model that has been developed over the past 18 months by the celebrated Handspring Puppet Company in South Africa. Operated by the synchronised movements of three men inside, it is claimed to simulate the movements of a horse with uncanny accuracy.

Meeting Morpurgo, who is now 63, it is easier to imagine him as a teacher than as his other previous self, an army officer. Yet this has some bearing on War Horse since it was at Sandhurst that he gained an insight into the life of a serving soldier, having always harboured a deep respect for people who fight for their country. A boarder virtually throughout his schooldays, he says that he survived by being good at rugby. At his public school (King’s, Canterbury) he was head boy. The clever ones at such institutions, he observes, go on to Oxford or Cambridge, while the nice but dim, in which group he places himself, quite often go down “the rugby route”.

Childhood, as he sees it, is a perilous thing. Miss a crucial turning here or there and you find yourself on the wrong road. Be lucky enough to have an inspirational influence in the form of a teacher, a parent or an older sibling, and the world opens up to you. The wastage, he implies, is almost as tragic as war. There is a hint of desperation in his analysis, and not just because it offends against an English sense of fair play – it is almost as if he is running from the man within him.

It is important to be moved, he concludes. “We mistake it in this country, and use the word sentimental instead. ‘Sentimental tosh’. It’s not, actually. The kind of art, music or literature that does something for me is when it gets under your skin and relates to you. We all tend to be indoctrinated to look at the big picture only, the huge statistics. Two million horses killed during the First World War. What does that mean? Nothing. But if you’ve known one of that two million, seen its life on the farm, seen it sold at auction, followed it, then it becomes deeply moving.”

One of the most influential figures in his own life did not emerge until the foothills of middle age. This was a neighbour in Devon who consoled Morpurgo when War Horse failed to win a Whitbread Prize, saying that prizes mattered less than books. He also said that Morpurgo would go on to write even better books. They had met a few years earlier when Morpurgo was fishing the Torridge and a man stepped from the river with a rod and introduced himself – it was Ted Hughes.

War Horse opens at the National Theatre: Olivier, South Bank, London SE1 (www.nationaltheatre.org.uk 020-7452 3000), on Oct 8

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