Michael Morpurgo's novel, War Horse, has become one of the great success story of the 21st century theatre - and it's about to get even bigger, with the release of a Steven Spielberg film. He explains to Claire Armitstead where the story came from, why he decided to put horses at the heart of it, and why - nearly a century on - we still talk about "the great war".

• This audio was updated on Monday 14 November to add a new introduction.