The true-life murder story of The Secret hit home for James Nesbitt.
He plays Colin Howell, respectable churchgoer and dentist, who started an affair with Sunday School teacher Hazel Buchanan, which led to the murder of their respective spouses, Lesley and Trevor, and which police initially thought was a suicide pact.
The murders took place in 1991, in Coleraine in Northern Ireland - where Nesbitt hails from.
"I've talked to many people who will have known Colin.
"I'm from Coleraine. He lived opposite my sister, they had the same cleaner, my sister went to some of Lesley's coffee mornings, which I only found out recently."
The dramatisation of the case is delivered in two feature-length episodes, tracing the story of Buchanan and Howell's affair, through to the pair's attempt to get away with murder and how their secret finally came to light.
Says executive producer Mark Redhead: "At its heart The Secret is a story of a love affair that starts discreetly in a church environment, and turns into a passionate and sexual adventure that leads to murder and then a bitter aftermath.
"It ultimately climaxes in the two central characters in the love affair turning on each other.
"That's a huge journey that unfolds over 18 years."
Playing Buchanan is Irish-born Australian actress Genevieve O'Reilly, whose credits include the comedy Episodes, Australian series Banished and an appearance in the forthcoming Rogue One: A Star Wars Story.
Nesbitt says playing the real-life couple had its challenges.
"It was hard in the sense that this was a love story, and a murder story.
"They were in love and so that had a big part of our story."
When the series screened in Britain in April, Nesbitt won praise for his credibly creepy portrayal of Howell, an outwardly devoted husband and father and enthusiastic churchgoer.
"It would have been easier if he had been incredibly charming. With Colin it was a bit more complicated because he wasn't."
When adultery turns to murder