But it was a welcome one. And a case of Froggatt unexpectedly getting exactly what she asked for when people inquired what she wanted to do once "Downton" ended.
"I jokingly kept saying, 'Oh, I don't know. Something completely different. Play a murderer or something,' " she said. "And then this script came along, and I just thought I've got to put my money where my mouth is because someone has just handed it to me on a plate."
In real life, Cotton (1832-73) was convicted of two murders, but she's suspected of as many as 21 — including three of her four husbands and 11 of her 13 children.
In "Dark Angel," Cotton has 11 victims, which is still a lot. Remember, Jack the Ripper supposedly only killed five people (although that unknown killer is also suspected of more).
"She was certainly on this spectrum of psychopathy," said Froggatt, who's also a "Dark Angel" producer. "But this wasn't a story just about a one-dimensional person that does terrible things. This is a story about a woman in that time period with no choices."
She doesn't excuse Cotton, but Froggatt said she tried to understand her. De Niro, on the other hand, said he can't understand Madoff at all.
"What he did is beyond my comprehension," said De Niro, who stars in "The Wizard of Lies" (Saturday, 8 p.m., HBO) as the man who swindled his clients out of some $65 billion. "So there's a disconnect somehow in him and I still would like to understand. I did as best I could, but I don't understand."
But we're talking De Niro here, so, yes, he got into the character. The HBO movie is based on the book by Diana B. Henriques, who interviewed Madoff after he was sent to prison. And during the filming of "Wizard," director Barry Levinson suggested that Henriques and De Niro improvise a similar series of questions and answers.
"This was not real hard for me," Henriques said. "I'm a journalist asking questions. But Bob is having to answer those questions extemporaneously out of Bernie Madoff's brain.
"And I made the vow right then and there to never take investment advice from Bob De Niro, just in case he's channeling his inner Bernie. It was so convincing."
Scott D. Pierce covers TV for The Salt Lake Tribune. Email him at email@example.com; follow him on Twitter @ScottDPierce.