Hollywood actor Luke Evans says telling the true story of the murder of four people was a "huge responsibility".
Evans, who was brought up in Aberbargoed, Caerphilly county, returned to Wales to star in ITV drama The Pembrokeshire Murders.
He plays Dyfed-Powys Police officer Steve Wilkins who in 2006 reopened two unsolved double murders from the 1980s.
"I just wanted to tell it right and show justice for the victims, which is the most important part," Evans said.
"This is a very serious, sad story where four people lost their lives and their families have struggled and suffered greatly because of it," he told BBC Radio Wales Breakfast.
"So you do feel a huge sense of responsibility."
The Pembrokeshire Murders has been adapted from a book about the case written by Mr Wilkins and ITV journalist Jonathan Hill.
In 1985 brother and sister Richard and Helen Thomas were shot at their remote mansion near Milford Haven, Pembrokeshire, before the property was set alight.
Then in 1989, Peter and Gwenda Dixon were shot dead at close range on the Pembrokeshire coastal path near Little Haven.
But it was only years later that microscopic DNA and fibres linked the murders to John Cooper, who was already in prison for a string of burglaries.
In 2011 he was jailed for life.
'Blew my mind'
The Dracula Untold star said he had not been aware of the notorious case: "I knew almost nothing about these murders, to the point where when I read what was a treatment two or three years ago… I couldn't believe what I was reading.
"So I did my own research into it and realised that the story was completely true - it hadn't been embellished, none of this was fiction and it sort of blew my mind."
He said being able to speak to Mr Wilkins while filming was invaluable: "Me and Steve had a dialogue almost every week for a few hours.
"We had a lot of conversations before we started shooting where I would speak to him and ask him, not just about the case - obviously that that was very important - but about things like how was it standing in front of John Cooper, having to interview John Cooper, having to deal with his family.
"You see both sides of the effect of these terrible crimes, you see what the aftermath of what it does to people and how they suffer and you meet Cooper's family as well.
"Steve has his own family and that also is played into the storyline very powerfully."
Evans said the only other time he has worked in Wales was when filming Visit Wales commercials: "Being Welsh and not getting to work in Wales very often - that certainly was an attraction for me," he said.
'Back to my roots'
"I've done them [the commercials] for a few years - one of them was about the coastal walks of Wales and our beautiful coastline... and then right in this beautiful place I was there back there, portraying a character and trying to find the killer of somebody who murdered people on this coastal path."
But he said he enjoyed playing a Welsh character: "To go right back to my roots with my accent and that was a really, really exciting to do.
The series, made by World Productions, the makers of Line of Duty and Bodyguard, finished filming just before Wales' first coronavirus lockdown.
"When we started The Pembrokeshire Murders it was January so we didn't hear anything really, and then just before we finished there was rumblings of this virus," he said.
"We were very lucky in a way, we wrapped basically on the Friday then on the Monday everything closed.
"So it was a big sigh of relief when we got to the final wrap of that day and it was very special."
The three-part series also stars Keith Allen, Owen Teale, Alexandria Riley, Caroline Berry, Oliver Ryan and David Fynn.
The Pembrokeshire Murders in on ITV at 21:00 GMT on 11, 12 and 13 January