Sinead Keenan almost turned down the chance to star in crime drama Unforgotten because she was “nervous” to replace Nicola Walker.

The Irish actor stars alongside Sanjeev Bhaskar in the ITV show's fifth series, but she initially refused to read the script.

“I said: ‘No, no. Thanks very much, no,'” Keenan reveals. “I said: ‘Who's going to be the gob***** to follow Nicola?”

Walker's much-loved character, Cassie Stuart, was killed off at the end of the fourth series of the hugely successful cold case murder drama.

Thankfully, Keenan did read it and 45-year-old stars as new DCI Jess James – and boss of DI Sunil ‘Sunny' Khan, played by Bhaskar.

Keenan – best known for playing Nina Pickering BBC supernatural drama Being Human – reveals: “What I was mainly nervous about was following Nicola Walker, because who wants to do that?”

“She's so brilliant and you almost have to forget about her (to do the role). I'm a different person. I'm a different actor. It's a different character. If I thought about it too much, well, I wouldn't have done it.”

The dramatic finale of the fourth series, by screenwriter Chris Lang, left viewers distraught as Cassie was killed in a random accident – a surprise to viewers, and Bhaskar too.

“I was shocked four times. I was shocked when I was told, I was shocked when I read the script. I was shocked when we filmed the scenes. I'm shocked when I watched it. So I kind of get why people were shocked,” says the 59-year-old.

For Bhaskar, who rose to fame on the BBC's Goodness Gracious Me, Sunny's grief for his former colleague and long-time friend Cassie was interesting to play as an actor.

He says: “Watching Chris's version of Sunny, he's a ‘steady Eddie', he's kind of a rock, he was there for Cassie. When Cassie was falling apart, he was able to hold it together, but we all have the capacity to lose it, to be really nasty or vicious.

“Chris basically thought: ‘OK, what is going to take Sunny to have that kind of outburst?' And that was an interesting thing to explore because I hadn't seen that side of him before.

“He was somebody who would, I thought, just be able to get up, push it down and go, ‘OK, I'll deal with that later.'

“I think grief particularly is incredibly confusing, you know, because it's not one thing. It's not just sad.”

Cassie's replacement on the force, Jess, has a rocky start. She gets some news in the first episode that blindsides her and she clashes with Sunny, so tensions are high.

“Sunny is grieving, the team is grieving, not just a colleague, but a friend,” Keenan says. “And she's doing a dreadful job.

“I can only imagine from Sunny's point of view, it's like: ‘Well I've lost one of my best friends, and this is who you choose to replace her with?'”

The new DCI's first case is the discovery of human remains in a chimney breast of a renovated house in Hammersmith, London. The team don't know how long the body has been there and Jess is initially reluctant to take on the case.

Just like the previous series, the structure sees several disparate antagonists who are possible suspects.

The story delves into the lives of characters including Jay, played by Rhys Yates (The Outlaws, Silent Witness), Tony, played by Ian McElhinney (Game of Thrones, Derry Girls) and Martina Laird (Summerland, The Bay).

Sunny's investigating team are back too, with DS Murray with Jordan Long as DS Murray Boulting, and Carolina Main as DC Fran Lingley reprising their roles.

Keenan describes Bhaskar as “a dream”, saying: “Honest to God, in between scenes, you sit down and fold your arms and just wait for him to entertain you with his anecdotes.”

Bhaskar says of his new co-star's introduction to the cast: “Sinead was in exactly the same position as her character Jess, that status coming into this kind of group of people that knew each other and got on really well, from which there was now a beloved member who was not going to be part of (it). That's a horrible thing to come into.”

Thankfully they bonded over a mutual appreciation of cheese (“Sinead's a cheeseaholic”) and Bhaskar sings the praises of his new – and former – co-star.

“I've been really lucky to work with incredibly strong and talented women throughout my entire career – and married one of them (Meera Syal).

“But on this particular series, Nicola and now Sinead are just brilliant at what they do and are humble about it, and there's no ego that comes with it.”

Lang says he was allowing the characters – Sunny and the team – to go on “exactly the same journey as the audience, as they deal with the loss of a person they adored, whilst also trying to process the brutal inevitability of someone else stepping into those much-loved shoes”.

In writing Unforgotten, Lang didn't want to just create a gripping murder mystery – he wanted to “hold up a mirror to the state of our nation”.

He said: “When I first started conceiving series five, towards the end of 2020, I knew I wanted to write about austerity and what effect it had had on our society over the previous 10 years or so. I could never have imagined that our country would now be leaning into austerity 2.0.”

Bhaskar adds: “I think Chris has rooted it in reality. It's fairly regular people that you see in society who did something terrible, which is probably what most prisons are full of. Most prisons are not full of serial killers but most detective dramas would have you believe most of them out there are.”

Unforgotten returns to ITV1 on Monday February 27