Ghosts and ghouls might scare us, but for any parent, true terror is the idea of any harm coming to their child.

It is this primal fear that writer/director David Farr has drawn upon in his directorial debut, The Ones Below, a psychological thriller about the entwining lives of two couples expecting their first babies, which premiered at the London Film Festival last year ahead of its wider UK release.

And as a father of two teenage children, Farr is all too familiar with the particular worries that only come with the birth of a child.

“The nightmare is clearer to you as a real thing, because the child is so helpless and never have you experienced something so helpless and so valuable,” the 46-year-old says. “No wonder then that some suspense or horror comes out of this environment.

“It’s an unusual moment when primal fears are at play.”

“I think we all go to movies and read stories to experience terrors that hopefully we will never have to experience, and this would be one of those, where we go through the ordeal in a safe place,” he adds.

Unfortunately for the doomed couples who live together in the same converted Islington house in The Ones Below, many of those fears are realised.

Initially, it is a fine-brush portrait of the domestic, and generally contented, lives of two middle-class couples: Clémence Poésy and Stephen Campbell-Moore as Kate and Justin, living upstairs; and Laura Birn and David Morrissey playing Theresa and Jon, living below.

But a tragic accident sparks a dramatic shift in the film’s tone and sets off a chain of Polanski-esque psychologically-disturbing events.

The idea came to Farr after his friend’s baby became seriously ill.

“We were talking about how terrifying it was to be in London, in an affluent family, and suddenly this terrible thing happens which no-one else can really understand.

“Suddenly you’re in a position of pure primal fear in the middle of very civilised north London, and we talked about how isolating it was.

“It really struck a chord with me, as I have two kids as well, with those early days of feeling very vulnerable. It’s magical, but very vulnerable.”

At the film’s centre is the relationship between two women, one living upstairs and one below, and how that friendship changes as the plot unfolds.

“There’s a creeping unease all the way through,” Farr explains. “But it’s purely psychological, because when the two women meet they get on.”

He adds: “It’s a fairytale. The princess in the tower sees another princess in the garden and goes out to play with her, and then the other princess doesn’t turn out to be everything she seems.”

Farr’s love of writing strong, female characters is also apparent in his six-part adaptation of John Le Carré’s The Night Manager, in which as writer he changed the sex of Olivia Coleman’s character Angela – Leonard in the book.

“Everyone knows that the recruitment policy of the intelligence services has diversified hugely,” Farr says. “It was also to give an actress of Olivia’s quality an interesting messy complicated role.

“I do like writing women and that’s obvious from the film, and I like writing women who aren’t just an extension of a man.”

It was not only the only change from the book that Farr made.

He has also transported the series from the 1990s to the 2011 Arab Spring, on the approval of Le Carré.

“He loved that. He found that very exciting. And after that, it was a very fruitful collaboration,” Farr says.

The Arab Spring could not be further from the comfortable north London affluence that the The Ones Below is set against.

Shot and located entirely in Islington, Farr said the choice of setting was entirely personal – he spent 10 years between 1993 and 2003 hopping from north London neighbourhood to north London neighbourhood, including Holloway and Stoke Newington.

His children were born at the Whittington and Homerton hospitals.

He says: “It was the arena of my child rearing, with buggies in Clissold Park, and playgroups in Highbury Park. It’s an area I know and love deeply.”

He adds: “I chose to shoot the film in Islington because it’s personal.

“I suppose another person could have made it Wimbledon, but they’re a quietly very fashionable couple, the upstairs couple, they do belong in north London,” Farr says.

“They’re a liberal kind of couple who would go and see this film, and they are decent, they have a quiet sense of style.”

The film is Farr’s first as a writer/director, though he was a screenwriter on 2011’s Hanna.

But his background is in theatre.

He grew up in Guildford to a German-Jewish mother, and fell into a theatre-directing career after university.

In the last 10 years, he has served as artistic director of Hammersmith’s Lyric Theatre and associate director of the Royal Shakespeare Company.

But now, aside from a few pet theatre projects, he says he will be focusing entirely on film and television.

His next film, HHhH, is already in post-production, and in the autumn it was announced that he is to be the writer for an upcoming £20million BBC drama about the Trojan war.

“I am more of a film lover, I always have been,” Farr explains. “I’m not like some other theatre directors who have come over to film and made slightly more theatrical films.

“There’s nothing wrong with those, they’re great, but my great love is existential French movies with very little dialogue.”

His minimalist approach is evident in The Ones Below, which mostly takes place in and around one house.

He says: “I don’t think I’ll ever do bombast. I hope not. I suspect I’ll never be a sweeping The Revenant style director.”

Adding that he takes influence from Alfred Hitchcock, he continues: “The risk was that it might appear theatrical but we negotiated around that.

“The house is a huge character in the film and it becomes her enemy, though it starts off very much as her friend,” he says.

“It was in my interest to try and make something very cinematic in a very claustrophobic environment. The story doesn’t need anything than other than what is there.”

The Ones Below is released tomorrow (Friday).