IN the dead of night in the Surrey countryside, bestselling children’s author Michael Morpurgo watched the horses skitter as the gunfire opened on both sides of the trenches.

The scene was so realistic and so disturbing that for a moment he forgot that it was a re-enactment of the chaos of the First World War, not the real thing.

“It was the moment in which the horses, lost in no man’s land, were panicking. It was extraordinary seeing 200 German soldiers in the trench watching these horses careering up and down. I was lost in it.”

He left the set of the forthcoming Steven Spielberg movie of his children’s novel, War Horse, feeling moved and upset: “I found it so depressing, so realistic, that you really didn’t want to be there. They had created this hellish battered wasteland scene in the middle of Wisley, right next door to the beautiful gardens of the Royal Horticultural Society. I drove away thinking, ‘I’m glad to have got out of there’.”

Morpurgo, 67, still cannot quite believe the acclaim War Horse has received since it was brought to the London stage four years ago, considering he wrote it nearly 30 years ago.

“The book was intended as an anthem to peace – a look at the universal suffering of the First World War as seen through the eyes of a horse. It was pretty much ignored when it came out, but fortunately the National Theatre picked it up and then Spielberg picked it up as well.”

A special edition has just been published and he realises there will be great excitement surrounding the film, whether it’s a hit or a flop, but knows, as the author, he’ll remain pretty anonymous.

“Do you know who wrote ET? I don’t. It’s a Spielberg film.”

The former Children’s Laureate and ex-primary school teacher remains one of this country’s most prolific children’s authors. He’s penned more than 120 books and won more awards than he can probably comfortably fit on the mantelpiece of his home in Devon which he shares with his wife Clare, and where he receives regular visits from his three children and seven grandchildren.

His stories are frequently set against a factual backdrop. His latest novel, Shadow, tells the story of an army sniffer dog in Afghanistan and the Afghan boy who befriends him, while previous books have tackled subjects such as the Israeli-Palestinian conflict through a child’s eyes, a boy’s struggle to survive after the 2004 Boxing Day tsunami, and the plight of a zoo elephant during the Second World War.

“I tell these stories because they matter to me. I’m a war baby, born in 1943, and that was my childhood. I was imbued with the consequences of war and the wreckage of lives and how the grief turns into anger and sometimes a wishful revenge. I don’t write thinking, ‘I’m going to change the world’.”

The War Horse film, due to be released next year, came about when a producer who had worked with Spielberg saw the play in London and contacted the Oscar-winning director about it. Morpurgo met him before filming started and visited the set on numerous occasions.

“Spielberg’s a wonderful storyteller and a kid. He adores stories and that’s what he’s best at. It’s extraordinary to meet someone with that kind of enthusiasm, utterly unspoiled. Then when I went to visit him on set he was clearly enthralled by the countryside. He was warm, kind and open and utterly without ego.”

• A special edition of War Horse is published in hardback by Egmont, priced £12.99.