Delia Balmer, the protagonist of ITV’s true-crime drama Until I Kill You, is a singular sort of person. She’s a “traveller”, not a “tourist”; a “female” never a “woman”, with the sharpest of tongues.

Balmer, played by Anna Maxwell Martin, is isolated. Living alone in a run-down flat in London’s Kentish Town in 1991, she is virtually friendless at the hospital where she works as a nurse, and her family lives overseas.

When she meets charismatic carpenter John Sweeney (Shaun Evans), it’s no surprise that she falls for him. But it’s a terrible mistake; Sweeney is a serial killer, and Balmer is soon trapped in an abusive and violent relationship.

Almost three decades later, Nick Stevens, who wrote The Pembrokeshire Murders and In Plain Sight, came across Balmer’s 2017 book Living With a Serial Killer, her account of her near-fatal relationship with Sweeney and subsequent distressing experiences with the police and criminal justice system. This became the basis of Until I Kill You, a four-part drama, which – unusually for the true-crime genre – tells the story through the eyes of a victim, not the serial killer or the police.

“Delia’s story was extraordinary,” says Stevens. “I reached the chapter describing – and this is not a very sophisticated way of putting it – losing her shit at the Old Bailey in the witness box. I was hooked.

“The first time I met her she was angry and agitated, and wouldn’t look me in the eye. She wasn’t hostile; it was just that the place she’s often at in her head makes it difficult for her to have human interactions. I realised how challenging she was as a personality, partly due to her post-traumatic stress disorder. I was thinking: ‘My God, I’m having a difficult time with her, but how would her [demeanour] as a victim impact on the treatment she receives at the hands of police and lawyers?’”

Balmer made a huge effort with her memoir, even attending creative writing classes. But, says Stevens: “The process of her manuscript being turned into a ghost-written book was a big disappointment. Delia was concerned that I do a good job.”

He describes a “close and sustained working relationship, bumpy but ultimately positive”. Knowing she was a “stickler for detail”, he told her from the start to “prepare for the inevitable distortions and compressions that come as a consequence of the adaptation process”.

The key to writing TV true crime, says Stevens, is to “stay true to the big narrative beats and not do anything that’s going to offend or distress” victims or their families. “The principal characters, family liaison officers, the main detectives – they were all people I’d met and are represented on screen more or less as they are in life.

‘Delia talked all through the screening at the top of her voice’

“I was upfront with her that I wasn’t going to show her the script until it was as-near-as-dammit the thing that would be filmed – she would’ve come back with a blizzard of stuff, and it would’ve hampered the process.”

When Balmer did read the script, Stevens recalls, she sent him a lengthy document, “My Critique”, which contained “everything from her experience of the courts and police to whether a mug was on the floor or table – that’s Delia”.

Stevens pitched to regular collaborator Simon Heath, CEO of World Productions, which made The Pembrokeshire Murders and In Plain Sight. Heath, who executive produces Until I Kill You, was “immediately struck” by the fact that it was Balmer’s own story: “The very nature of a serial killer story is that the victim will be dead. Here was a victim who survived, and that made it utterly unique, and much more worthwhile telling than another story about a botched police investigation or [going] inside the mind of a killer.”

Maxwell Martin, who worked with Heath on Line of Duty and The Bletchley Circle, and Evans were the first choices for the leads. “It was one of those fortunate and rare occasions when the people you want are both available and want to do it,” says Heath.

Balmer is a challenging role: Maxwell Martin has to be funny, feisty, confrontational and, at times, dislikeable, but also brave and resilient in the face of physical and mental torment. “She was the perfect casting,” says Heath. “With a true story, there is a duty of care that goes far beyond the words on the page. I knew Anna would be great with that, and so it proved.”

Evans (Endeavour, Vigil), continues Heath, provides the charisma that makes Balmer falling for Sweeney so believable: “Shaun has that charm in spades. But when Sweeney shows his dark, violent nature, Shaun is able to do that with a minimal flick of a switch. It’s chilling and terrifying.

“There’s a point in the film where you’re buying into the relationship and you hope things work out – then you have to catch yourself because you know what actually happens.”

Balmer was keen to visit the shoot, which largely took place in Swansea, doubling for 1990s north London. “We made a deliberate choice not to have her on set when any of the grim things were happening, which were the days when Delia wanted to visit,” Stevens says. “She was there on the most neutral day imaginable – which she was a bit narked about – but we were just trying, understandably, to be sensitive.”

Is Balmer pleased with the finished drama? Yes, but on her own terms. “We had the most unusual screening I’ve ever attended,” recalls Stevens. “Delia talked throughout the whole thing at the top of her voice, saying, ‘That never happened’ or laughing out loud.”

Stevens, who regularly sees Balmer for dinner, adds: “This has definitely been a positive experience for her, not least because of the money she received for her book – that’s been life-changing. She’s travelling more than she did before.”

Why we love true crime

Simon Heath

‘There’s the old saying that “fact is stranger than fiction”, and so often real life throws up stories that we could never conceive. Therein lies the appeal; the uniqueness of real-life stories. Having the understanding that the underlying facts of a drama are true can make it that much more arresting for an audience.

‘It would be a stretch to call it the drama of reassurance, but generally the true crime that gets on to the screen offers some kind of resolution, with some kind of justice.’

Nick Stevens

‘Touchstone books for me in my early 20s were The Executioner’s Song by Norman Mailer [about the execution of Gary Gilmore] and In Cold Blood by Truman Capote [a quadruple murder in Kansas]. I wouldn’t have been able to tell you why they fascinated me as a would-be writer, but that intersection of an author’s voice more traditionally associated with drama being applied to factual material I found very exciting.

‘Speaking from my perspective now, I have a tough time [writing] fictional stories; I’m always tripping over myself, thinking “this feels so fake”. So what Simon was talking about – the reassurance of knowing something’s true –  is massively freeing. I’ve been given the skeleton of something and I can introduce my desire to write “fiction” in the gaps between. That just works for me.’

Until I Kill You and Until I Kill You: The Real Story, an accompanying documentary, are on ITVX.