Downton Abbey fans will be hard pressed to find any similarities between Anna Bates and Joanne Froggatt's latest role in PBS' upcoming miniseries Dark Angel.

And that's just the way the Golden Globe winner wanted it.

"People kept saying to me, 'What did I want to do after Downton? I jokingly kept saying, 'Oh, I don't know. Something completely different. Play a murderer or something," the actress told reporters Tuesday at the Television Critics Association winter press tour. "Then this script came along and I thought, 'Oh, I better put my money where my mouth is.'"

In the two-part drama, Froggatt plays the real-life figure Mary Ann Cotton, the first female serial killer in England. Known for murdering three of her four husbands and 11 or her 13 children, Cotton was eventually hanged to death.

The challenge, naturally, of the role came with finding a way to make Mary Ann sympathetic to viewers. "Her story starts off as a story of desperation, really," This wasn't a story just about a one-dimensional person that does horrible things. This was a story about a woman in that time period who had to make desperate choices."

Read more: Golden Globe-Winning ‘Downton Abbey’ Star to Become ITV’s Victorian Poisoner

Those choices start after Mary Ann is forced to bury her first six children, who all died of natural causes. "She's almost driven mad with grief," said Froggatt. Her husband loses his job and she struggles to find a way to make ends meet for her family. That is, until she decides to kill her husband to receive his insurance money.

"The first time she does it, you could argue, it's to feed her children," Froggatt said. "Once she's done it once, it's now something that has changed her. … She's lost a part of her humanity after doing that deed, and therefore the next time it makes it easier."

The jump from Anna to Mary Ann was a big one, especially considering that Froggatt started work on Dark Angel just two days after wrapping the final season of Downton Abbey. "It was lovely, actually, to have something to focus on, to go straight on to, because I think I would have felt the weight of the end so much more if I hadn't had something to focus on straight-away," she said.

It also helped that Froggatt wasn't alone in her move to darker material post-Downton. "[Co-star Kevin Doyle's] next role is also a serial killer, so me and Kevin were sort of swamping notes," she said with a laugh. Froggatt even had a fellow Downton Abbey alum on set: Series director Brian Percival also helmed Dark Angel, which was written by Gwyneth Hughes and inspired by David Wilson's book Mary Ann Cotton: Britain's First Female Serial Killer. "It was great because Brian and I had worked together so we had a shorthand," she said.

It's no coincidence that, like Downton Abbey, Dark Angel will air on PBS' Masterpiece. As an associate producer on the ITV miniseries, it was Froggatt who brought the drama to Masterpiece's executive producers.

"I didn't have say in all areas, but I was able to OK certain things, and it also meant I had a right to push my nose in and approach Masterpiece and PBS and say, … 'I really think this is a Masterpiece show,'" she recalled of her summer meeting with the exec producers. "It was a just a project I felt extremely passionate about."

Suffice it to say, it sounds like Froggatt is enjoying her post-Downton career thus far. "I want to challenge myself, and I want to do something different every time," she said. "I get very easily bored, so I like things to just keep me occupied."

Dark Angel premieres later this year on PBS.