'I wanted to play a killer' Downton darling plays murderer Mary Ann.
AS Downton Abbey neared its conclusion, Joanne Froggatt, known to fans as the compassionate lady's maid Anna Bates, was often asked what role she would like to play next.
"I jokingly said a murderer," reveals the actress. "And then the script for Dark Angel came through."
The two-part drama for ITV tells the tale of Mary Ann Cotton, who was hanged in 1873 for the murder of her seven-year-old stepson, though it's believed she actually killed many more, including three of her four husbands (biographer David Wilson believes her total victim tally to be 16 or 17).
Yet remarkably, no one really knows her name.
"She was from the North East and I'm from Whitby, which isn't that far away, and I hadn't heard of her," admits Froggatt, 36.
"It's quite a fascinating story in that respect, because she's the first recorded female serial killer in Britain that we know of."
She has a theory as to how Mary Ann got away with the killings for so long, and doesn't have the notoriety of someone like Jack the Ripper, who terrorised Victorian London.
"I learnt that female serial killers tend to poison or overdose or smother people, clean and tidy ways," says the actress.
"Men like to make a mess and it's often a sexually-driven crime. For women, it's more to move up, in either social or financial status."
This was the case for Mary Ann, who moved from town to town in the Sunderland area, marrying and creating new families before killing them and taking what money she could.
"Had she stayed in one place, people may have become suspicious that all these people around her had died," notes Froggatt, who's appeared in TV series Bad Girls, Life On Mars and See No Evil: The Moors Murders.
Then there's the simple fact she was a woman.
"People didn't really think a woman was capable of it," the actress points out. "Even when she was imprisoned, there were high-profile people who wrote letters on her behalf saying she can't possibly be capable of it."
In Dark Angel, we meet Mary Ann when she's 25, and has already suffered the natural loss of four children.
When her first husband, Bill, is injured and can no longer work, they have no way of paying the rent or feeding the family. But he does have life insurance, which was a new concept at the time.
"It wasn't her idea to insure her first husband's life, it was her step-father's [George Stott, played by Alun Armstrong]," reveals Froggatt.
Mary Ann poisons Bill with arsenic, which was readily available to buy and left no visible scars.
"Once she's done that, she's not able to stop," Froggatt adds. "And yes, by the end of it, she's a very different person than when she started. She certainly chooses a path that most people wouldn't and there's obviously a reason for that, and it has to do with the way she's wired."
Mary Ann was married four times, once as a bigamist, but the love of her life was Joe Nattrass, played by Jonas Armstrong.
Downton Abbery ended last Christmas after six series, in which time Froggatt won a Golden Globe for her performance.
There's talk of a Downton film, although it's yet to be confirmed and remains to be seen which of the cast would return.