If there is one character in "Downton Abbey" who is faultlessly and morally upright, then that would have to be Joanna Froggatt's character Anna Bates. Froggatt did justice to the character of Anna, so much so that she even won a Golden Globe and SAG awards for her masterful portrayal.

So when "Downton Abbey" wrapped up its sixth and final season, and Froggatt was offered another role in a period piece called "Dark Angel" set in the United Kingdom, fans would be in shock, since Froggatt would shift from playing the goody-two-shoes Anna and into real-life Victorian serial killer Mary Ann Colton.

The role was something that Froggatt could not resist, and the challenge excited her.

"I got this script sent to me and I said, 'I don't know. I quite foresee going to the States for a bit.' But as soon as I read it I just went, 'Oh my goodness, it's amazing.' It's such a fantastic script," she told Deadline. "It's kind of exactly what I've been saying I wanted to do. It's written by Gwyneth Hughes who's a fantastic writer. We see the character at 25 and she suffered the loss of, through natural causes, four of her own children. Then through the course of her life you see her sort of slip away bit by bit and become this murderer and this sort of psychopath. It's just a fascinating character, and really very different from Anna - I just couldn't resist it."

"Downton Abbey" creator Julian Fellowes has earlier revealed that they would finish the show open-ended, but Froggatt said that they would still find closure in the case of Mr. Green, who brutally raped Anna a few seasons back.

Both Anna and her husband Mr. Bates have already been sent to prison after Mr. Green was mysteriously murdered, and fans do not know who is responsible for his death.

"We do see the mystery behind Mr. Green's death resolved. Obviously how it's resolved I can't tell you, or whether it's good or bad for the Bates I can't tell you, but it's resolved," she said. "We get to the bottom with what exactly happened, and as always, there's quite a lot of drama for Anna and Mr Bates in season six. There are always bits of happiness there for them as well, but it's never easily attained, so we sort of carry on in that vein to finish off."