Southcliffe: the most harrowing drama on TV

The Kent and Canterbury Hospital is a busy infirmary in the centre of one of the UK's most famous cathedral cities. This afternoon, however, the distinction between reality and fiction is becoming a little blurred. On the second floor, real paramedics and nurses rub shoulders with actors dressed as the white-coated medical staff of the fictional coastal town of Southcliffe.

Filming of an emotional scene is under way. Chris, a serviceman recently returned from Afghanistan, is trying to visit an injured soldier friend, but finds he is no longer in the same bed. In a quiet way, it's intense stuff. "Not to use a wanky actory word," says Joe Dempsie, who plays Chris, "but with my character, there's a real 'journey', a real range of emotions."

A spellbinding and terrifying piece of television storytelling, Southcliffe ramps things up a notch from recent shows like Broadchurch and The Fall, maintaining an incredible suspense across its four-hour duration. Written by Tony Grisoni (Red Riding) and directed by Sean Durkin (Martha Marcy May Marlene), it gently drifts forwards and backwards in time, visiting and revisiting its characters, progressively uncovering their relationships, digging a little deeper on each occasion.

At the centre of the drama is a lone-gunman shooting, in the vein of Dunblane or, more recently, west Cumbria. For Durkin, however, while the shooting is the catalyst for the drama, it's not what it's all about. "It's more the tool by which we see people dealing with grief," he says, "rather than about the tragedy in cultural terms."

In Southcliffe, we're left in no doubt who committed the crime – another returning serviceman, played by Sean Harris (who has in his career inhabited plenty of extreme roles, ranging from Ian Curtis to Ian Brady) – but we're intrigued to find out exactly why he did it. Fittingly, for an actor playing a TV news reporter who is called to the scene, it is Rory Kinnear who best articulates the conflicting emotions around such an event. "I guess as with all kinds of crime, your first reaction is shock," he says. "A lot of people shy away from thinking about the perpetrator because they think it may reduce their empathy for the victims. As a journalist investigating a crime, you're always looking for what led to it."

Joe Dempsie agrees. "When a gunman kills themselves you don't find out why they did it. This drama goes some way to answering some of those questions. In life, people from the area say things like, 'Yeah, they were always a bit weird'. But I don't think it's ever that inevitable."

Key to the mood of suspense and mysteriousness is the location of the filming. In Southcliffe, the passage of time is articulated by the certainty of the shipping forecast, but the landscape and weather is wonderfully ambiguous. Kent provides an attractive landscape, but not outstandingly so; low-lying mists can seem enchanting one moment, threatening the next. It's a neutral kind of setting where it won't take too much to tip the mood into darkness. "It's such a rich landscape, you can understand how it might get overlooked," says Durkin. "It's quite bleak but quite beautiful, and quite average as well. It captures all of these feelings." Such is the unforced, naturalistic mood that Durkin has created here, it allows the film's big-name actors to move with a great equality within it. Kinnear, who played M's aide Bill Tanner in the last two Bond movies, is full of praise. "Everything's as believable as possible," he says. "There's no point doing a piece as atmospheric and meditative as this one if it's too drama-laden."

For an actor like Dempsie, who's been playing an increasingly prominent role as King Robert's bastard son Gendry amid the swords, dragons and full-frontal nudity of Game Of Thrones, it's certainly been a change. "In Game Of Thrones you're a small cog in a big machine – the director's got a million other things to think about, not just your performance," he says. "They hope you've got your part down so they can concentrate on the 10 horses that are going to enter shot. I like that. But I also like an actor-focused piece, where the director is working with you. It's a small crew, and that helps you get to quite dark places."

Undoubtedly, Southcliffe is just that kind of dark place. Is it too much to ask of a TV audience, to get them to tune in for four hours of realistic, often horrifying drama? "I don't think so," says Eddie Marsan, who plays a grieving dad. "I think the rhythm of a four-parter like this allows you to reach depths on a more subtle level. I think it's something TV can do that film can't. It's not a rollercoaster."

Miraculously, none of the actors look as if they bear the scars of any psychological trauma. As filming concludes, Kinnear is off to film sitcom Count Arthur Strong, which he confesses will be nice, "after nine weeks in this mood". But otherwise, the way his cast have been able to commit to a harrowing scene and then withdraw has been a wonder to Durkin. "I don't think anyone will say it was gruelling," he smiles. "I think we had a great time. It was a fun shoot, believe it or not."

Southcliffe starts early August on Channel 4