In driving sleet and sub-zero temperatures, actors and scores of extras negotiated ground that was knee-deep in mud, criss-crossed with barbed- wire-trimmed trenches, and hazy with smoke from guns and explosives. Artificial bodies and limbs lay partly buried in the mire, and the pace was frenetic, says director Gillies MacKinnon.
A scene in which 80 men were cut down by machine-gun fire in a matter of seconds had not long been completed when, at 11am sharp, everything and everyone stopped. The two-minute silence that followed was "the most moving, the most eerie experience I've ever known," recalls one crew member.
It had been scriptwriter Allan Scott's idea to mark the day thus. For one of the UK's most successful and prolific film writers and producers, a Hollywood player of the first order and a man dubbed the "Richelieu of Wardour Street" by observers of his high-level machinations on behalf of the film industry, the scenes being recreated in Glasgow were too powerful to ignore.
"There we were, surrounded by all the iconography of the First World War," Mr Scott, 54, said last week. "It had rained for weeks, and the mud was like nothing you have ever seen. I thought there was no way we could not mark it." The cast and crew agreed. "We caught a glimpse of the reality of that war in a way that few people have ever had," Gillies MacKinnon said.
Regeneration, adapted from the first of the Pat Barker trilogy which culminated in the Booker Prize for The Ghost Road, is the latest venture in the celluloid rediscovery of the events of 1914-1918. Lord Attenborough's In Love and War - for which Scott wrote the script - opened last week, a film of Sebastian Faulks's Birdsong is also proposed, and Scott is working with Sir David Puttnam on a A Very Long Engagement, another Great War project.
Scott believes the current interest in the First World War is because "we're the last generation able to touch it through our grandparents". Regeneration was a book he believed in from the moment he read it, shortly after it was published in 1991. He immediately acquired the rights and wrote a screenplay "on spec" - an unusual act for a writer who has been in constant demand since his script for Nicolas Roeg's classic Don't Look Now. He took it to the BBC, and he knew who he wanted to direct it; the young Scottish director MacKinnon, who has since won critical acclaim and numerous awards for his film Small Faces.
But time passed and the rights went to MGM who were keen to develop a film about Siegfried Sassoon, the aristocratic poet, war hero, and anti- war protester. It is the intriguing cerebral relationship between Sassoon, played by James Wilby, and the army psychologist, Dr W H Rivers, at Craiglockhart War Hospital in Edinburgh in 1917, which provides the fact blended seamlessly with the fiction in the book.
But Regeneration is about more than Sassoon. It is, according to Scott, about how Rivers, "managed to make sane men mad enough to go back to war and how he lives with that. Or rather he doesn't. He becomes shell-shocked through treating their shell-shock".
And it is also about Billy Prior, the clever, working-class soldier promoted to officer class and rendered mute by his experiences, who is Rivers' most hostile patient. In him, Pat Barker has created one of the most complex and chilling characters in modern fiction. Jonny Lee Miller, "Sick Boy" in last year's mega-hit, Trainspotting, brings him to life in the film.
Scott, 54, a charming and occasionally indiscreet interviewee, feared the loss of all this potential to Hollywood. "Occasionally Gillies and I would meet up and cry into our beer but he was always optimistic that we'd do it," he said. "About 18 months ago I heard the rights were available again, I had the script and so I said to Gillies 'I'm ready if you are.'"
The film, now in post-production and due to premier at Cannes in May, stars Jonathan Pryce as Rivers, famous as a neurologist and social anthropologist in academic circles before the War. He is the pivotal figure around whom Sassoon and Prior move. Robert Graves, Sassoon's great friend and the man who persuades him to seek help for his pacifist leanings, makes a fleeting appearance along with Wilfred Owen. Owen was at Craiglockhart in late 1917 being treated for neurasthenia or "shell-shock" but was not a Rivers patient.
It was Owen's encounters with Sassoon - he hero-worships and is secretly in love with him according to the book - which resulted in the body of work which marks out Owen as the war poet for most people.
"What is fascinating," says Scott, "is that Owen hadn't written a war poem until he met Sassoon. He tells him that he thinks that poetry is 'something to take refuge in' from all the ugliness. Sassoon tells him to face facts. It was from October 1917 until he died [in November1918] that he wrote the war poems."
The film will introduce a new generation to the poetry of the Great War. But its inclusion presented Scott with problems. He had wanted to use "Dulce et decorum est" at the end but it was pointed out that most of his audience would not know what it meant. (In Owen's words, "The old Lie: Dulce et decorum est/Pro patria mori" - It is sweet and honourable to die for your country). Since Regeneration is intended as big box-office rather than art house, it must appeal to Mr and Mrs Iowa et al, so Scott had to find a way of explaining Dulce et... early in the film so that the audience would not realise it had been "set up". How was it done? "Wait and see."