“I STILL have moments where I go: ‘F***, John Malkovich is in my show.’”

The writer Sarah Phelps doesn’t really do quiet. She comes into the conference room in the BBC’S New Broadcasting House, hair a bird’s nest halo, and starts talking at a mile a minute speed. About drinking absinthe cocktails (five of them in one night: “They really hurt when you wake up, don’t they?”) and politics and fear and, oh yes, the head-turning fact that she has a proper Hollywood star appearing in her new TV show The ABC Murders.

She still can’t quite believe it. “I was chatting to him on set: ‘Yeah, yeah, vineyards, wine, I like wine,” she begins and then veers off to let us know in a stage whisper what was going on in her head at the same time: “Con Air. Con Air! I’m talking to John Malkovich!”

It’s possible she wasn’t the only one. Filming in Bradford City Hall, she says, the building’s staff would pass Malkovich dressed in period costume and trainers doing stretches on the stairs and “literally reverse back up and peer at him. It was really funny to watch.”

Malkovich, star of Being John Malkovich (bien sur), Dangerous Liaisons and, yes, Con Air, is playing Hercule Poirot in The ABC Murders, the fourth of Phelps’s idiosyncratic (some might say wilfully nose-thumbing) adaptations of Agatha Christie.

It’s also her first go at Poirot himself. Phelps hadn’t been interested before, she admits. “I hadn’t wanted to do a sleuth. I really hadn’t because I didn’t want someone to come along and solve things. I didn’t want anyone to have any answers. I wanted everything to be murky and for no one to have answers. Nobody was going to come along and make it all right. Absolutely nobody. You’re on your own. This is it. Deal with it.

“I’ve always felt with the Marples and Poirots, they come along and tell you what’s happening and at the end it’s all tied up with a neat bow and order is restored. And I really didn’t want to do that.”

And yet here she is introducing us to her version of Poirot, one of the most famous literary characters in crime fiction? As Phelps admits herself. “You could probably show anyone in the world that silhouette and they could say: ‘That’s Poirot.’”

So how then did she go about making the character work for her. “I thought: ‘Who’s the private man behind the silhouette? If we all think we know who Poirot, then who the hell is Hercule?”

Ever since she gave us And Then There Were None back in 2015, Phelps has been praised and pilloried for her vivid and at times liberty-taking reboots of the Christie canon. The ABC Murders is the latest star-studded adaptation (as well as Malkovich, the cast includes Rupert Grint, Freya Mavor, Tara Fitzgerald and Shirley Henderson) in what is becoming something of a tradition in the BBC Christmas schedule. I about the only thing that is traditional about them of course.

Phelps is not interested in fidelity to the original text. She wants to tell a good story. That is what matters to her. And so, in The ABC Murders Poirot’s policeman friend Arthur Hastings shuffles off the stage early in proceedings. “The first thing was [I thought] I’m getting rid of Hastings. He gets on my nerves. I wanted Hercule to be friendless. In the book he has lots of friends, all these endless, useless coppers who pop up being useless.

“I don’t want to do that. I want to see the world through this man’s eyes. And also, I want to see him as somebody who is a stranger, somebody who is living in a very difficult England.”

Well, yes. The ABC Murders, filmed by director Alex Gabassi in a style we might call murky realism, offers a vision of England that could not be further than the cosy, even twee, murder-with-afternoon-tea stereotype that sometimes clings to Christie’s name.

Here we are back in 1933. The British Union of Fascists is on the rise and on the march, raising banners and verbally and sometimes physically attacking anyone they might think is different.

Doing her research, Phelps says, “I found my way to some really, really dodgy websites which I never want to go to again and I found the marching anthems of the BUF and they really were going to purify our nation and purify our blood and our maidens and then we’re going to hoist our banners over the blood-stained streets. F****** hell. Jesus.”

Watching The ABC Murders, it is difficult to avoid seeing the parallels to the current political mood and moment. Phelps doesn’t begin to try to. “The language they used is absolutely no different from the language of Brexit and Trump. ‘Expel the alien, reclaim the country.’”

And, of course, in 1933 Hercule Poirot was an alien. “Canonically he is a refugee. He walks out of an absolutely fire-torn Belgium after the German invasion in 1914 to come and solve The Mysterious Affair at Styles, this person who is somehow the epitome of Englishness in certain imagination. No, he is not. He is an outsider, l’etranger. He comes from somewhere else and he carries that with him.”

What does that make The ABC Murders? Not just a whodunnit (though that remains one of its pleasures inevitably). For Phelps, true drama needs more than just a gathering of the suspects in the drawing room.

“No one gives a s***,” she argues. “Because you’re not seeing a human being and you’re not seeing someone’s moral compulsion. You’re not seeing what drives them, you’re not seeing interiority; their private selves. When they wake up at four in the morning in a muck sweat what are they frightened of?”

Because in the end the why matters just as much as the who. If not more. Even in crime fiction. Sarah Phelps knows this.

The ABC Murders begins on BBC One on Boxing Day at 9pm and runs over three consecutive nights. For an interview with Freya Mavor see this weekend’s Herald on Sunday.