Nicola Walker and Sanjeev Bhaskar on the chemistry that makes Unforgotten's cop duo tick

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“There is a deep, deep sadness running through these stories,” Walker says. “And when you’re doing stuff that’s really dark, you can stay in that place. So, I’d be really serious all the time if it wasn’t for Sanjeev. It’s his fault if I’m not always serious.” Bhaskar feigns offence. “My fault? I’d say it’s my credit,” he says. “There are scenes in Unforgotten that are so intense, horrible and tragic that you have to find a way to relate to each other as human beings again.”

We are in a high-ceilinged drawing room in a dilapidated former country house in Buckinghamshire, where they’re shooting series three. As well as the pathology lab, the building houses the cluttered incident room where the cold case team consider pictures of disinterred bodies. Exteriors are shot in the grounds, where their co-star Neil Morrissey is presently wandering.

The drama attracts a high-profile supporting cast. As well as Morrissey, Trevor Eve, Bernard Hill, Gemma Jones, Wendy Craig, Douglas Hodge and Mark Bonnar have all appeared, and Bhaskar and Walker speak with awe of Tom Courtenay’s Bafta-winning performance as Eric Slater in series one. “It’s the only time I’ve seen the crew all break into a spontaneous round of applause,” says Walker.

Actors like Courtenay come, in part, for the quality of Unforgotten creator Chris Lang’s writing; scripts that slip beyond the confines of procedural to address the psychological state of the nation. “The first season asked if a crime is any less a crime because it happened a long time ago,” says Walker. “And Cassie felt absolutely not. The second ruminated on the concept of justice. When you go back and look at how the people who committed a crime got to that point, when you see that they suffered horrific abuse, then justice gets a bit more complicated.”

She has a theory that, thanks in part to the Jimmy Savile and Rolf Harris revelations, we live in a time when the past, and television’s past especially, is untrustworthy. The British were ready for a show that seemed to redeem the sins of the 70s and 80s. “We grew up with Jimmy Savile on our televisions,” she says. “Chris was fascinated by what it was like for the people who knew someone like that – people who loved him and believed the lies that he told. What was it like when they found out there was this whole other person?”

Walker had been wondering the same things herself. “It felt like all the people we grew up seeing on TV and were told we should respect were falling like dominoes,” she says. “It was a strange period; every time I read the newspaper there was someone else. I’d been told as a kid they were talented, important people, but they were living a lie. A lot of those men from the 70s had this completely different life.”

Now the USA is also being obliged to face the misdemeanours of its entertainment industry and audiences are picking up on Unforgotten. “The first two series are going out in America,” says Bhaskar. “People there are tweeting me about it, and they’re caught up in these stories. I think it has a dramatic weight that other whodunnits may not carry to the same degree. It’s because our characters aren’t heroic… we don’t save the day.”

Much of the drama’s appeal rests in the interactions between the two leads. Stuart is Khan’s senior, but there is more going on than the usual detective and sidekick relationship. Morse’s private life was a mystery to Lewis, and vice versa, but Stuart and Khan know each other. “I haven’t really seen it before in a cop show where you’ve got a man and a woman who really, really care about each other, respect each other and look out for each other. They provide a level of intimacy for each other they don’t have at home…”

Walker, 48, who is married to the actor Barnaby Kay with whom she has a son, says she has always been lucky with her co-stars. “Just think of Last Tango in Halifax. You walk on and it’s Derek Jacobi and Annie Reid and Sarah Lancashire. You’re in a really good place.”

Born in east London and a graduate of Cambridge University’s Footlights, Walker is an acclaimed stage actor, with an Olivier Award in 2013 for her performance as the mother, Judy, in The Curious Incident of the Dog in the Night-Time. Bhaskar is 54 and married to Meera Syal, with whom he has a son, and is from west London. After starting life above a launderette, he went on to work for IBM before giving it up for comedy and, latterly, making documentaries for the BBC.

Walker says their pairing worked from the beginning. “I was a fan of his work, and as soon as I sat down next to him at the first readthrough, which was when we met, I knew. They had written our names on little cardboard tags and they were next to each other, in this huge room with the executive producer, writer, director, all the other actors. And as soon as I sat down next to him, I thought, ‘This is going to be absolutely fine.’”

“I didn’t think it would work at all!” says Bhaskar, deadpan. “I’m still waiting for it to click in in any sort of shape, way or fashion.” Walker, who has known Bhaskar too long to be taken in, laughs. “OK,” he says. “Nicola is one of the most amazing people I’ve worked with.” In fact, Bhaskar admits, when they are not acting together he takes a proprietorial pride in her work. “I watched the series of Last Tango she did after Unforgotten and there’s a bit of me that goes, ‘That’s my friend, being fantastic.’”

Last Tango in Halifax, The Split… Walker has a standout drama CV. Bhaskar has made his name in broad comedy. “Comedy is seen on the outside as being easier,” says Bhaskar, “because there’s a very definitive reaction you need from comedy, and that’s laughter. From a drama, you don’t have people kind of waving audibly in order to know that you’ve got it right. It’s a physical, tangible thing.”

Many of Bhaskar’s shows, comedies such as Goodness Gracious Me and The Kumars at No 42, have emphasised his Asian background. “I was kind of surprised and impressed that Chris Lang had thought, ‘Yes, why can’t it be an Asian cop who is the partner?’” says Bhaskar. “And then they didn’t feel the need to load it up with references to justify there being an Asian bloke there. That ‘the corner shop didn’t work out so well’ thing. Khan is a real copper who applied for a job once and got it. And he happens to be part of this story.

“I feel just as strongly about the way that women have been portrayed in the past, and the older generation as well. In the end, it comes down to good storytelling. You can tick every kind of politically correct box and still end up with a box of crap.”

It’s a trope of even the most right-on modern crime series that a woman, often young and attractive, must be murdered, and it bugs Walker. “There have been jobs that I haven’t gone to the audition for because you think, ‘I don’t want to play someone who is violently murdered in episode two.’ Or ‘I don’t want to be in something that’s all about young women being murdered.’ There are some amazing dramas being made that use it as the dramatic hook, and say interesting things about us culturally and have really good stories, but not everything we make tells the story well.”

Series two of Unforgotten began, like series one, with the discovery of a male body. It also saw the relationship between Khan and Stuart almost tip into romance. “Chris played around with us, and the audience,” says Walker. “He said, ‘Right, here’s the bit where, in most shows, they’d bring in the sexual tension,’ and he brought it in and then dismissed it within one scene. Brilliant.”

So, what can we expect of series three – more national redemption? “It’s partly about asking, what you do when you can’t understand the motives,” says Bhaskar. “How do you deal with people like that? Because you have to look for a motive, otherwise it’s just random. It about not dismissing a past crime by saying, ‘Well, that’s just evil.’ It’s about trying to understand it because otherwise, how do we understand anything in the future?”

Bhaskar turns to Walker. “Sorry, I do jabber on a bit. I realise that.” “It’s all right,” she says. “I love it when you talk."

Unforgotten starts on Sunday 15th July at 9.00pm on ITV