Putting on a brave front: Behind the scenes of the BBC's epic adaptation of 'Birdsong'

On the banks of a dirty, swollen tributary of the Danube, a picnic is under way. Under bamboo parasols and on heavy rugs lie the remains of a sumptuous spread: baguettes, fruit, cheese, cake, lemon barley water and a wine of rather pricey-looking vintage – 1906 Château Les Alberts Bordeaux. But rather than Budapest in the early 21st century, we are in Amiens 101 years earlier.

A callow English textile manufacturer, Stephen Wraysford, played by Eddie Redmayne (My Week with Marilyn), distractedly whittles wood as the river gurgles by. A Frenchwoman, Isabelle Azaire (Clémence Poésy, aka Fleur Delacour in the Harry Potter films), sits poised and erect in the shade. She is the young French wife of Wraysford's host, a local merchant. The honk of geese can be heard through the reeds, although the soundtrack promised by the script ("the hum of bees... wasps feeding hungrily on a tree weighted with overripe pears") is nowhere to be heard. Also not pictured: an "interminable heat". It's a cloudy, overcast day.

"The thing to remember is that in never rains in Hungary in July," a jacketed cameraman wryly notes as crew members scurry around uncoiling cables and replanting foliage. Presently, the rain begins to patter again, pooling mud in walkways. The squelch of boots can be heard all around. Given the story presently being assembled on this film set – one of love, but also the glutinous gore of trench warfare – these conditions are, in a way, crudely apt.

It is summer 2011, and in a wood on the edge of Budapest, it is day 33 of the 43-day shoot for Birdsong. The crew on this BBC/Working Title adaptation of Sebastian Faulks' bestseller set in and around the First World War are working hard to recreate the France of both the narrative strands on which screenwriter Abi Morgan (The Hour, The Iron Lady) has focused: drowsy Amiens in 1910, and the horror of the Western Front in 1916. Morgan opted early on to leave out the novel's third element, a near-contemporary depiction of Wraysford's granddaughter. k

"The modern-day stuff was a brilliant framing device," acknowledges the writer, who is currently attracting fresh acclaim for her script for artist-turned-director Steve McQueen's Oscar-tipped film Shame (reviewed in main paper). "But we decided that for the economy of a TV script, and because we were working within two 90-minute episodes, we wanted to focus on the intensity of before and after the love affair between Stephen and Isabelle."

Hence, "before", we have the elegance of today's picnic scene, all fine food and even finer wardrobes. In her jam-packed trailer, a couple of hundred yards from the Danube, costume designer Charlotte Walter rifles through rail after rail of Edwardian-era clothing, painstakingly sourced in Paris and London, and some recreated from photographs in her own family's albums. "Stephen arrives from England dressed in greys. But soon he's wearing cream suits – a young woman in France can have that effect," she smiles.

Similarly, for "after", when Wraysford is a battle-scarred captain in the infantry, staring down the barrel of the battle that will produce the worst single-day casualties in British military history, the producers worked hard on ensuring the soldiers' uniforms looked the part. Hobnail boots came from Poland, leather-capped officers' wristwatches were fashioned by the props department, and woollen shirts by the dozen were "pre-distressed". Walter applied such volumes of fake gore to the outfits that, "I had so much blood under my fingernails I looked like a mass murderer." Over the course of filming the battle scenes, Redmayne wore through eight uniforms.

Walter also located an original, officer-style greatcoat from Burberry, and was granted access to its archives – a process eased by the fact that Redmayne has modelled for the brand. Advised by military experts, Walter and her small team also created their own regimental insignia, to avoid any criticism from First World War experts and to be sensitive to veterans' groups. Advisers from the Imperial War Museum have offered pointers on soldiers' movements and drill, and medical experts have been on hand "to tell us what 90 per cent burns would look like", says the venture's producer, Lynn Horsford.

With equal attention to detail, trenches were excavated by the production team in a sunflower field a few miles away. The deep spying and attack tunnels dug under the battlefields – in which Faulks sets much of his wartime action – have been ingeniously replicated in a nearby studio.

All of which, it seems, was easier to do in Hungary than France. Yes, concedes Horsford, "It's sad that we're not in France," but as ITV found when they shot Julian Fellowes' upcoming Titanic here the month before the BBC production, "you get a tax break" by filming in the former eastern bloc state. Plus, Horsford adds, some streets in Hungary are time capsules. "It's doubled for [old world] France many times," she says – most recently for the Guy de Maupassant adaptation Bel Ami, starring Robert Pattinson, out in March. Furthermore, director Philip Martin (Wallander and the Bafta-winning Mo, about Mo Mowlam) is on familiar territory: he shot 2008's Einstein and Eddington here with David Tennant.

The scene being filmed today is the moment where Faulks' hero and heroine make their first fleeting, physical contact. In the formal, stuffy, stuffed-shirt atmosphere of the Azaire household, Stephen and Isabelle's growing feelings for each other have hitherto been played out in stolen glances and tremulous gulps. But as they boat along the river in that "interminable heat", their ankles graze together. It's an electrifying and ominous moment, and not just because of the sign we see on the edge of the river: we are in the département de la Somme.

"These scenes between Clémence and Eddie are all about the danger their characters are walking into," offers cinematographer Julian Court. "And by putting them under the trees and using the shadows and having them almost silhouetted, we're trying to visually suggest that jeopardy."

"I want to recalibrate what is sexual," Martin adds. "We want to take the audience back to a time when a touch was an extremely dangerous and provocative and erotic thing."

For Isabelle and Stephen, that touch of skin on skin lets the genie out of the bottle. Their affair is incendiary, entirely based around explosive passion. In one memorable passage in the book, Faulks writes in graphic detail of the "sex act" (to use the normal newspaper euphemism) the Englishman performs on the Frenchwoman.

"As a young man reading the book, for me that was an incredibly erotic scene," chuckles Redmayne between takes. The actor, now 30, was an adolescent at the time he read the 1993 novel, "and a lot of my friends were blown away by it. We have a responsibility to this book, but in some ways doing that scene [properly] is also a massive responsibility."

Poésy had no time to duck that responsibility – she and Redmayne filmed the scene on her first day on set. Was that a deliberate choice on her part? "No, no, no, no," the Parisian actress, 29, says emphatically. "I looked at the schedule and I was, like, 'Really, are we going there already?' But then there was that thought of, OK, well at least it's out of the way. And of course it wasn't – we had to go back to it every single week because we never finished it that day!

"But it needed to be done properly. And it's weird because I try to avoid those scenes. I had a policy against things like that as I had a bad experience." (Aged 18, Poésy appeared in a French film in which, against her better judgement, she shot a topless scene.) "But I think every actress says that, then you grow old, then you really don't give a shit."

'Birdsong' is the epic book that, for almost 20 years, was the failed film. As Geoffrey Macnab detailed in a March 2009 feature in The Independent, a screen adaptation of the novel by Faulks – deputy editor of this newspaper when it launched in 1990 – has been a slow train coming. "Over the past 16 years," Macnab wrote, "hundreds of thousands of pounds have been spent on commissioning scripts, optioning and re-optioning the source material, hiring lawyers and scouting locations. There are no particular heroes or villains from this epic process. This isn't a cautionary Hollywood parable about some faceless studio calling the shots. It's an everyday tale of the British film industry, where projects not infrequently spiral off into some development hinterland whence they struggle to emerge."

Iain Softley, Sam Mendes, Rupert Wyatt, Joe Wright; Ewan McGregor, Ralph Fiennes, Damian Lewis, Paddy Considine, Eva Green – a host of directors and actors have been involved with various attempts to film the 500-page book. Abi Morgan reckons she's the 11th writer to attempt a screenplay (and she's had more than one crack at it). Poésy says that even she had previously considered the project – when she was clearing out her email inbox last year she found an old version of the script that she'd been sent.

In most key respects, Birdsong as a movie is a no-brainer – as Macnab put it, it could be "a film that, if it lives up to its potential, should carry us all away with its sweeping historical narrative and tragic romantic undertow". So what, I ask Horsford, was the problem? "It's a big question," she smiles. As she observes, "everybody adores this book", but no satisfactory script for a single film ever materialised. "It was either too expensive, or too unwieldy." (Spoiler alert: readers who have not had the pleasure of the book may wish to skip the rest of this paragraph.) "It has a very unconventional trajectory, and it doesn't have a conventional happy ending. So you can imagine the reaction in Hollywood: 'Hang on a second – he doesn't get the girl?'"

The most recent attempt to realise a big-screen version was headed by director Rupert Wyatt (Rise of the Planet of the Apes). He got as far as scouting locations in France, and his producing partner Damian Lewis – too old to play Wraysford but with decent bankability – was in the frame to portray one of the other officers. But when that too collapsed, Morgan and Juliette Howell at Brit-film powerhouse Working Title had a rethink. Writer and producer reasoned that, rather than trying to shoehorn the big, tricksy novel into a single film, it might make more sense to do it, as Horsford puts it, "on a bigger canvas, over a longer period of time – two 90-minute [television episodes]. And the moment that decision was made, everything fell into place."

Morgan found that the story was further unlocked for her by the 2009 passing of the 111-year-old veteran known as the "last fighting Tommy". "With the death of Harry Patch, I wanted to ensure that the passion and the poignancy and the historical significance of Birdsong was brought to the screen and to another audience," says the writer. "I felt very strongly that they're a generation that are no longer around to tell that story. Sebastian Faulks had done it so brilliantly and it was just a wonderful opportunity to ensure that that appalling war and the nature of the conditions they fought in was documented."

Martin shares this desire to give the historical resonance of the century-old conflict a contemporary relevance: as well as making his cast watch documentary footage of the Somme in 1916, he screened for them Restrepo, the Afghanistan conflict documentary made by photographer Tim Hetherington, who was killed last year in Libya.

"You see dust being kicked up by horses' hooves [in the old footage]," says the director, "and I'd always had an image of the First World War that was very wet. But suddenly you were seeing that it was very hot and dusty and much more like Helmand province than just rain-drenched, muddy trenches. That really informed the way we wanted to do it. We wanted to go into the research – then emerge from that research with a new way of looking at the War."

Regarding the casting, Redmayne was top of Horsford and Martin's list, having impressed them in the 2008 TV adaptation of Tess of the D'Urbervilles. Says the producer, "We were trying to find an actor who could convincingly go from a young, rather naïve innocent 20-year old to someone completely different – over the span of the war it's as if he's aged another 20 years."

For the French roles, Martin sought French actors, "so that Stephen's connection would be a real one, if you like", suggests the director. "Clémence is an incredible actress, and she has a haunting, mesmerising beauty that [as Isabelle] you feel she could capture Stephen's heart."

"She's just luminous on screen," adds Horsford – with the added bonus that, via her appearances in the Harry Potter franchise and Gossip Girl, Poésy "has quite a wide following. She appeals to perhaps a younger audience. I know we're going to get the literary intelligentsia watching because of the book. But on BBC1 you want a wider audience. And Clémence has that scope."

Poésy, long a part-time resident in London, says that even though Birdsong doesn't have the cultural presence in France that it does in the UK, she's well aware of its iconic importance to her British friends – which made her protective over Isabelle. "I'd never fought for a character like that before 'cos I knew how my friends, especially the girls, were looking at her – they were looking at her as very modern in her choices and who she is and the mystery that's left... That's what people love about Isabelle. So I really stood up for her quite a bit."

For Redmayne, too, his character became precious. "Stephen is an isolated man, damaged as a kid, who is now being rewarded with love and passion for this woman. He's being opened. But Abi contrasts the two stories, the love and the war, so his stillness in the trenches isn't opaque. You see where it comes from. There's almost a mystery thriller quality to it – how has this man come to where he is?"

On the evidence of episode one, in all the right ways Birdsong doesn't compete with the First World War as epically depicted in Steven Spielberg's new film War Horse. It couldn't – the latter is a big-budget Hollywood blockbuster – but neither should it. This is a story of love, and of friendship (between Wraysford and the men under his command, notably tunneller Jack Firebrace). Under Martin's acute direction, stillness and silence – of the courtship, of the tunnels – say more than whizz-bang action. For all the widescreen carnage of the Western Front, Birdsong is a story thick with claustrophobic emotion.

Redmayne, for his part, went as far as he could into the research. He and Joseph Mawle (who plays Firebrace) made a recce to the Flanders battlefields. They were shown into First World War tunnels that had only recently been discovered. Since they were dug into chalk, as soon as a light was shone on them, "it was like being in an igloo". Deep underground, their guide showed them a patch of wall. There was a poem written in pencil, "as if it was yesterday. It said: 'If in this place you are detained/ don't look around you all in vain/ but cast your net and you will find/ that every cloud is silver lined... still.' It was extraordinary," the actor marvels. "We were the fifth people down there in 100 years. And there it was, hope in the most horrific of circumstances."

For Redmayne – whose next project is another French-set drama of passion and conflict, Les Misérables – making Birdsong was a gruelling endeavour requiring deep-down commitment. "We were trying to make something on the scale that the story deserves. And doing it on a limited budget. So everything was pushed to the limit.

"Now," he smiles, "obviously that's a millionth of what anyone had to go through in the reality of the War. And it was useful to remember that on set," he concludes with a laugh, "when I was about to go, 'Hey, where's my chair?'"

'Birdsong' airs on BBC1 in late January