Line Of Duty and Motherland star Anna Maxwell Martin reveals there were moments her co-star genuinely terrified her in her latest serial killer drama.

Anna plays Delia Balmer in upcoming ITV true crime thriller Until I Kill You, which sees Vigil actor Shaun Evans play killer John Sweeney, who is currently banged up in prison for two canal murders. Sweeney tried to kill his girlfriend, agency nurse Delia – and very nearly did, in a brutal attack with an axe in 1994, but she miraculously survived. He had previously confessed to murdering an old girlfriend, Melissa Halstead, chopping her up and throwing her body in a suitcase in an Amsterdam canal. He was also convicted for murdering another ex, Paula Fields, in the same way, throwing her body into a London canal, and police suspect he killed other women.

The real John Sweeney and Delia Balmer in Mexico

Anna says: “I found a lot of watching Shaun bring John to life quite impactful because it was very realistic. The violence when he attacks Delia on the steps… yes, it was very real. And that makes you fully inhabit the role and realise how traumatic and terrible that must have been for Delia. Then watching Shaun in court was pretty terrifying. He was genuinely frightening. I can’t imagine how that must have been for everyone in real life. I thought, ‘God, this is a lot’.”

Anna admits that the choice of who was cast opposite her for the thriller was “a real deal breaker”. She says: “It was really important to me who was going to play John Sweeney. I spoke to the director and we sort of agreed, ‘Let’s not get anyone who’s a d***head’. It would just be too difficult, someone who goes in on themselves, someone who sucks air out of the room. But Shaun is just a brilliant listener. You want everyone to feel comfortable and not have a miserable time at work.”

Shaun, also known for his role as a young Inspector Morse in ITV’s Endeavour, nearly rejected the part. He says: “Obviously you’re the antagonist and I was kind of repelled when I read it. But then I thought that’s interesting territory – but I had trepidation. It’s a delicate balance because these are real people.” Both Anna and Shaun decided not to meet Delia before filming, though she visited the set, with Shaun listening to Sweeney’s voice and keeping copies of Sweeney’s gruesome drawings of his victims in his dressing room.

There was one particular scene that Shaun describes as like being “punched in the gut”. He says: “It’s the scene where Delia has been tied to the bed and it’s about his manipulation of her and it was a very harrowing moment and I thought, ‘This did actually happen’.” In the scene, Sweeney has tied Delia to the bed for several days and threatened her with a gun and a knife, warning her that if she screamed, he would cut her tongue out.

Anna adds: “That scene was less impactful for me because when you’ve got to be tied to a bed completely naked, your head goes to a really practical and technical place.” Anna and Shaun also reveal that Delia, now 74, visited the set and struggled with certain details, unhappy with the placement of some props and some of the costumes. Anna says: “It must be extremely challenging to watch your life played out.”

The drama deals with the way female victims of male violence have been dealt with by the police and by the judicial system. Writer and executive producer Nick Stevens adds: “The first time I met Delia, I went to her flat, which has a lot of artefacts in it from those days, including the bed that Sweeney made them. And one of the first things she did as she was talking to me to demonstrate what had happened was throw herself on the bed, arms out, legs out. Then she took me into the kitchen, took a knife out of the drawer and repeatedly went for her stomach to demonstrate what he’d done with that. So that gives you some insight into where Delia’s coming from. In terms of the detail, it’s all in there.”