THIS is Agatha Christie so there’s a body on the floor. A glamorous woman in a black evening gown. There’s a pool of red around her head and her arm is outstretched. And look at the jewels, the diamonds on her neck and her fingers. Could it be a robbery? The maid says she saw a man leave earlier, a much younger man, the victim’s lover. The woman’s cat is licking its paws, cleaning them of the last few spots of blood.

It’s a typically macabre but glamorous Christie set-up and the central crime in one of her most famous thrillers The Witness for the Prosecution. However, the theory behind the latest dramatisation – a two-part BBC special to be shown on December 26 and 27 – is that there is more to Christie than the puzzle and the twist.

For a start, Emily French, the glamorous woman who has been murdered, is much more than a cardboard archetype: she’s a feminist, complicated and demanding and a little scandalous. But Sarah Phelps, who has written the new BBC version of Witness, was also determined to bust some of the persistent myths about Christie.

Before she took on the job, says Phelps, she thought she knew what the world’s best-selling whodunit writer was all about and it was sitting round a cosy fire and watching a Miss Marple. But Phelps has a different theory. Christie is the opposite of cosy: she’s subversive, she’s challenging, and she’s shocking.

The portrayal of Emily French by Kim Cattral in the new dramatisation, which also stars Toby Jones, certainly brings out this more daring edge. Emily is a wealthy widow whose comfortable world has started to suffocate her and she thinks one way out is a relationship with a younger man.

“For her age and her time Emily is very much a feminist,” says Cattral, who is best known for Sex and the City. “First of all she goes out at night hoping to find exciting partners and new friendships. This story is set in the aftermath of the First World War when a whole generation of men were lost, so it’s hard to find a man anyway, and most of the men that are left are either very young or much older. When she meets this gorgeous, vulnerable young man he is different from anyone around her and her interest is piqued. This is not simply about an older woman preying on a younger man, it’s more than just her gratification; she wants an adventure.”

Cattral, who was born in Liverpool but moved to Canada with her family when she was a baby, says she was very familiar with the original short story on which The Witness for the Prosecution is based – in fact, she was looking into a possible theatre production in New York when she heard about the BBC’s plans and was cast as Emily French.

“I’ve always loved Agatha Christie,” she says. “My mum was a big murder mystery buff and there was always an Alfred Hitchcock or Agatha Christie in the house.” However, the key to the new BBC version of Witness, she says, is the fact that Phelps has dug under the source material. “We have made it our own,” says Cattral, “while at the same time been very faithful to this amazing writer who has created some terrific parts for women – and that was not fashionable.”

Phelps herself says that her aim was to go right back to the original short story, which was published in 1925, and bypass the other adaptations, such as the stage play written by Christie herself and the 1957 film starring Marlene Dietrich and Charles Laughton and directed by Billy Wilder. “I kind of wanted Christie’s first thought,” says Phelps. “It’s very early on in her writing and creative life.”

Having dramatised And Then There Were None last year, Phelps was also starting to develop her own theory about Christie and The Witness for the Prosecution seemed to fit. “The short story is much more subversive than the play,” she says. “Before I adapted And Then There Were None, I had never read an Agatha Christie but because I didn’t know the books or have any previous relationship to it, I came to it with an entirely different point of view, sort of raw which meant that I felt very shocked and surprised by what those stories actually are. You have no idea who you are in a room with: they sound like you, they look like you, they sound respectable and you pull away the veneer of their respectability and Englishness and what you get underneath is somebody who is completely different and terrifying.”

When it came to the story of Witness specifically, Phelps also saw a story about otherness. “I see a story about sex, money and murder, otherness, set a few years after the end of the First World War – this is when one party of society is dancing on tables, kicking their legs up, drinking Champagne and having a right old time. But the other half of the population is starving – they came back from the war broken and promises were made to them and those promises were not kept. Men with unseen injuries – injuries to the mind – came back to their tiny rooms with wives and children and they were strangers to them. It’s like a fuse – you put a light to it and it explodes.”

Phelps says this approach was typical of Christie – the desire to tackle whatever was going on in society at the time – an analysis that Christie’s great-grandson James Prichard agrees with. Prichard runs the Agatha Christie estate and has spent a few months reacquainting himself with many of his famous relative’s novels and what struck him was the fact that, far from being a writer of nostalgia, Christie always set her novels slap bang in the middle of the present day.

“It struck me just how set in their time the books are – she doesn’t get the credit for that,” he says. “If you read one of her 1920s books, it’s set in the 1920s – the 1950s books are set in the 50s. In the 60s, you can see the way she, as an older woman, is flustered by girls going down the King’s Road in short skirts.”

Prichard says his aim in allowing dramatisations such as Witness is to remain true to the feeling rather than the letter of the source material and he is entirely open to a freer and more adventurous take on the stories because that’s how he thinks of his great-grandmother.

“People always think of her as an old woman as if she was born a grandmother,” he says. “But she was a young woman for a large part of her writing career and she was incredibly adventurous. She was a single mother, she went on all those archaeological digs, and some extraordinary places in the Middle East that you couldn’t go to now. People call it cosy crime but I don’t think there was anything particularly cosy about my great-grandmother – she was very forceful.”

That’s how Phelps likes to see her too. “She is fierce and subversive,” she says. “There’s a snobbery about the genre but also it’s productivity and having a big audience and being a populist author but in her best work, there is an overarching umbrella of murder mystery, but she has a subversive take on what she’s writing about. You have these huge establishment monoliths, the law, the army, the church, all of these things that are supposed to be about a stable, decent society and she does something that tweaks it. It’s done ever so quietly and sneakily and with the most deft touch – but she takes all these things and spins them.”

The Witness for the Prosecution is on BBC1 on December 26 and December 27 at 9pm