In August 2010 details started leaking out online. Someone resembling Steven Spielberg had been spotted in the Co-op in Yelverton on the southwestern edge of Dartmoor. "He didn't look familiar to the area," one local said. "He looked like he was going to buy the Co-op rather than buy a sandwich." It was later suggested – but not confirmed – that Spielberg had bought a pint of semi-skimmed milk.

Later a man answering to the Hollywood director's description –"dressed in a black leather jacket and his signature cap" – was seen striding across the moorland near Sheepstor on Dartmoor. A grainy photograph of Spielberg kneeling and squinting in a Devon meadow surfaced on the web. Locals reported fruit carts and hay wagons popping up behind hedgerows. Crew members were seen tarting up barren trees with extra twigs and leaves. One holidaymaker quoted on This is Devon's website summed it up as "many unusual comings and goings".

The English location shoots for War Horse – in Devon and Wiltshire – were as secretive as you could hope they would be when you are working with hundreds of extras and crew. Locals wanting to be extras were told they were auditioning for a film called Dartmoor set during the first world war. Anyone who inquired directly about War Horse was told: "Can't say anything. It's a secret."

Ever since news had broken that Spielberg was filming War Horse, there were high hopes that this would be good news for Devon in particular. The book's author, Michael Morpurgo, used his home village of Iddesleigh, near Winkleigh in north Devon, as part of the novel's setting, so it was assumed the county would feature.

Dartmoor was selected as a frontrunner early on. Its rolling landscapes are perfect for the film's early scenes. The production team decided, however, that Castle Combe in Wiltshire would stand in as the "nearby village" local to the farmhouse, where the livestock auction to sell Joey (the horse of the film's title) takes place.

Dartmoor's role ended up being more prominent than initially envisaged, explains production services manager Fiona Francombe: "War Horse was coming for a couple of days. But once Spielberg had been down there, he decided he wanted to do more. Getting a big production like that is a huge thing for any area. You don't get bigger than a Spielberg film coming out of Hollywood. It's fantastic for raising the profile of the region because it's going to be seen by millions. It's the biggest advert imaginable."

When Spielberg came to town in the summer of 2010, the West Country rumour mill went into overdrive. He arrived at Exeter airport in a private Boeing 737 and it was immediately reported he had come straight from Chelsea Clinton's wedding. In fact, he was in Devon on the day of Ms Clinton's nuptials, on a 450-acre farm near Widecombe-in-the-Moor, shooting scenes near Bonehill Rocks. Holwell Farm, owned by Philippa and Sebastian Hughes, was used as the production crew's base: 400 cars and 20 lorries took over the site ahead of a 4am start.

The couple spoke fondly of Spielberg inspecting their piglets and Dartmoor ponies: "He loved the animals and we found him to be a very easy person to be around. His entourage was quite formidable, but because he is such an important person, the security around him reflects that."

Other Dartmoor locations included the villages of Meavy and Widecombe. An isolated Grade II-listed building, Ditsworthy Warren, near Sheepstor, serves as the Narracott family's farmhouse: the action begins here. It is to this remote farmhouse that Joey, the horse who is the star of the film, returns after having been bought at auction by Ted Narracott (Peter Mullan), father of Albert (Jeremy Irvine), the boy who becomes devoted to Joey.

The casting of extras in Devon and Wiltshire was done with maximum discretion. It was one audition hopeful who first reported on a blog that he had been told the film was called Dartmoor when he turned up at Princetown village hall to have his photograph and measurements taken (reportedly more than 1,000 hopefuls auditioned for the Dartmoor scenes). At such castings, it's all about the picture: you need to look like someone from the olden days. The portfolio is shown to the producer, who decides on "the best faces from that period".

Bracken Vernon-Jelier, a PR executive from Yelverton, blogged that she had seen the horses' trainer at work for the scenes shot on Dartmoor's Ringmoor Down. The horses had been taught to "fall down 'dead' when a gun is fired". She added: "The numbers of people working on set were astronomical: from people in charge of stuffing muddy clods into stone walls, to breaking off gorse bushes only to balance them back again five minutes later."

In Castle Combe, there was a similar quasi-military operation. Cast and crew took over all the accommodation within a 15-mile radius, and 300 extras and 30 horses were recruited.

Recruiting extras was a slightly different business to what it had been in Dartmoor: being offered extra work is an occupational hazard of life in Castle Combe, which has been a popular choice for period filming, having featured in Lark Rise to Candleford and Stardust. "Many have done it before and realise it's not as glamorous as it seems," says location manager Lee Robertson, who spent several months scouting for locations before it was decided Castle Combe suited Spielberg best.

Everything was conducted in immaculate detail. All the village's white window frames had to be repainted for a start. "White is a problem," Robertson says. "It stands out. Directors of photography don't like the glare. We have to dull it or paint it a different colour. You have to find a balance between what you want and what the person who owns the house wants." (It saves the film company time if they do not have to restore everything to its original condition.)

The through road had to be closed "because we wanted to lay a temporary surface that was basically mud. It would not have been tarmac in the early 20th century." Meanwhile, the crew worked to achieve a look of two different seasons – spring and autumn – for two scenes. "The team wired flowers into all the foliage, then took them out again for the autumn scene."

Adrian Bishop, a member of the Castle Combe parish council, says it was an enjoyable experience for the village: "They were here for three weeks. The first week they set up, then they did a five-day shoot –they had scheduled seven, but got it done in five – and spent a week clearing up. They converted the village back to 1914 and did it very accurately.

"They spared no pains in getting that right. They took down all the modern lamp fittings and signs, they covered up postboxes and notice boards. They had quite a lot of animals in this one. But we've had Dr Dolittle, so we were quite used to that."

Did he see the famous man himself? "Ah, yes, Spielberg was knocking about. He seemed fairly reclusive and had guards looking after him. I believe he did go into the local pub and have a couple of drinks." The only disappointment locally is that the village choir, shot for the "men heading off to war" scene, has apparently ended up on the cutting-room floor.

But in a film as ambitious as this, there will always be casualties. Spielberg was clear from the outset that this would be an epic – "a timeless story about the sacrifices of love". This meant there was pressure on the British locations to be perfect. Landscape is even described as "the third character" in the story (the others being the horse and the boy). Luckily, England did not disappoint.

The director was impressed by Dartmoor in particular: "I have never before, in my long, eclectic career, been gifted with such an abundance of natural beauty as I experienced filming War Horse. With two and a half weeks of extensive coverage of landscapes and skies, I hardly scratched the surface of the visual opportunities that were offered to me."

Perhaps you'll be back, then? We'll keep a landing strip ready for the Boeing just in case.