Michael Morpurgo: He is a terrific storyteller who understands the need to immerse himself in the tale

Excited is absolutely the word to describe how I'm feeling at the moment. It's beyond any author's wildest dreams to have someone like Steven Spielberg making a film of your book and the production is turning out to be an extraordinary gathering of British and American talent.

When Spielberg and Kathy Kennedy get together things really do happen. The cast list is wonderful; we have Germans playing Germans, French playing French and some marvellous British actors.

I think he's going about it all in the right way by simply getting the best actors that he can. He's an American man but this is very much a British and European story.

I simply cannot believe the speed with which things have moved on. Often these sort of things take years to get off the ground, but with War Horse it's taken a matter of months to get a film up and running. It's utterly astounding.

I met Steven a few months ago. It was a real privilege to sit across the table from someone who simply loves stories. Like all good storytellers he knows that if you want to do the job properly you have to immerse yourself in it.

War Horse has grown almost organically over the past five years. It started off as a book I wrote for children almost 30 years ago. It was translated into one or two languages and stayed in print thanks to the patience and support of my publishers. It sort of just about clung on in there but it was what I called an "almost book".

Then there was this extraordinary moment when Tom Morris at the National Theatre was looking for a vehicle for a troupe of puppeteers from South Africa that he had come across.

He wanted to produce a play where puppets, instead of being subsidiary characters, would be the stars. His mother came across my book and handed it to him. The book becoming a film was a similar happy accident. It was thanks to a Dreamworks producer who happened to see the show.

The book is a way to understand the universal suffering of war – not just the First World War: it could be any war. Spielberg, of course, has tackled war in many of his films but he hasn't touched the First World War yet.

What I have found intriguing is that, by and large, Americans don't know much about the First World War. I think they know the Revolutionary War, the Second World War, and Vietnam. These have all been extensively written about and made into films.

But the First World War doesn't feature as prominently. Yet America lost around 118,000 men in that conflict. It was a very extraordinary and admirable input into European history. I will be really happy if the film helps Americans comprehend that. I'm hoping they do something special, but even if they do something half as special as the play I'll still be happy.