David Morrissey thought "trippy" new drama The City And The City would be unfilmable

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In The City and The City, Morrissey's character Inspector Tyador Borlú lives in the dilapidated, chaotic, vibrant city of Beszel. But Beszel overlaps and mingles with an entirely separate city: the rich, slick and sterile Ul Qoma. The inhabitants of each city must consciously "unsee" the inhabitants of the other, or they will be punished by the all-seeing secret service Breach.

As Morrissey concedes, "It's a difficult show to describe. But once you embrace it and get in the world, you're really in it."

A citizen of Beszel may not visit the shop next door, because it's in Ul Qoma; a man in Ul Qoma cannot pick up a stray frisbee that has come from a child playing in Beszel.

"You don't acknowledge it, you don't have any interaction with that place at all," Morrissey explains. "And that's how it is. And it's sort of always been that way."

The concept comes straight from China Mieville's novel The City & The City, and adapted for TV by screenwriter Tony Grisoni. This four-part drama tells the story of a murder investigation that stretches across the two cities after the body of an American student who was living in Ul Qoma turns up on a street in Beszel.

Inspector Borlú is determined to find out who killed her, and why.

"I get sent a lot of cop shows," Morrissey says. "It's great that I'm six foot three and I get sent a lot of options, and cop shows are our staple, aren't they? So you get them and sometimes go, 'Oh, I think that's OK.' But this one I thought, 'Thank god for that! This is totally different.'"

But while it might be a totally different sort of cop show, Mieville's world contains a certain truth about our reality.

"What I like about [author] China, and I think there's something a little Black Mirror-y about it, is the fact that he takes things that you totally understand, are totally familiar with – and just exaggerates it"

He adds: "I don't think anybody who lives in any metropolis has to look too far to recognise that we deny people who live in our streets and our cities all the time. We don't look at the things that are going on. We live in high rise, or even on the street we walk past people all the time.

"So he's using that idea but militarising it, and putting it into a structure."

However, this "trippy" world is particularly hard to put on screen. How do you visually represent the idea of "unseeing"; how do you show two cities with completely invisible boundaries?

"I did start thinking, how the hell are they going to do this?" Morrissey admits. "And then I met [the director] Tom Shankland, who I'm a huge fan of, and he had a vision, and you just jump in. Any job is a leap of faith."

Production took place in Liverpool and Manchester, using the Victorian gothic buildings of the industrial revolution for Beszel which sit "cheek by jowl with these big glass structures" used to represent Ul Qoma. "Unseeing" is represented by a blurring of everything but the city our characters are currently in.

And, in an echo of the division between East and West Berlin that is still visible from the air – with one side lit up in warm orange and one in cold blue, a relic of Soviet vs Western street lighting systems – Beszel and Ul Qoma are bathed in different coloured light. It's a useful visual distinction.

Grisoni has however taken one liberty with the novel: he has given Morrissey's character a wife.

However, she is only introduced to us in flashbacks, because Katrynia (Sherlock's Lara Pulver) is missing. It seems she tried to illicitly cross from Beszel to Ul Qoma; now her whereabouts is uncertain.

"What Tony Grisoni has done is he's added something to the narrative, an emotional heart to it," Morrissey says.

"What I like about China's writing actually is there's something detached in his writing. He's slightly on the outside looking in.

"What Tony has done is he's given my character a real emotional heart that means that this case that he's investigating has a personal element to it. Why is he doing this? Why is he following this case so passionately? And it's to do with his ex-wife."