Even when they’re playing horrible villains, actors aim to understand them. Actress Joanne Froggatt, best known as the earnest maid Anna from “Downton Abbey,” says she tries to reach inside the evildoers she portrays.

“I am constantly fascinated by human behavior, which is probably why I’m an actress,” she says.

“Just constantly fascinated with why people do things, how people get through certain events in their life, why people behave in certain ways, good or bad, what gets them to that point in their life? Is it nature or nurture? I just find that whole unanswerable question absolutely fascinating. Anything with extreme behavior sort of fascinates me.”

The question has intrigued her before in “Liar” and “Angela Black,” and she proves that again in her latest foray with the thriller, “Sherwood,” now on BritBox. Froggatt admits she plays the “baddie” in this series, which is based on actual events in England following a turbulent miners’ strike. When two ghastly murders occur in this mining community, it shatters the already-disordered district and results in a massive manhunt.

“To play the antagonist is always fun,” says Froggatt. “I don’t think I approach it differently if it’s based around a real subject matter or a sensitive subject matter. I do think there’s a sort of added responsibility to it when tackling a project like this. I don’t think it affects how I prepare for the job as an actor, but it certainly has a gravitas that you sort of carry with you that I think is important.”

Kevin Doyle, who played the butler-turned-footman Molesley in “Downton Abbey,” costars.

“What intrigues me is not so much who done it but — and I think this show particularly explores — that is WHY. Why it was done. Whenever I’m presented with somebody like a killer, for instance — and I’ve played a few in the past — I’m always trying to humanize them, to make them understandable to audiences. And so that’s what I look for, that’s what attracts me to particularly good dramas is to try and identify with — even when terrible things are done. Why have those terrible things been done?” he says.

For costar David Morrissey, who plays the detective chief inspector on the case, it’s also the variety he seeks.

“The challenge is what keeps me going, I think. That’s why I’ve never been in a series that goes on for years and years and years. I like playing different characters. You look at ‘The Missing,’ ‘The Walking Dead,’ ‘Blackpool’ (all in which he starred), three totally different shows and asking something different from me. That’s what you’re there for: to take the skin off the onion,” he says.

“It’s not working down a coal mine, not doing three jobs to keep my family together, not being a soldier, or on the police force, it’s an absolute privilege when I go to work. Even if it’s scary or frightening, it’s a feeling of being alive. I love it, it’s great!”

The same goes for Lesley Manville, who costars in “Sherwood” and earned an Academy Award nomination for “Phantom Thread.”

“What informs my choices and has always done is extremes of characters. Different characters from what I’ve played before and different ends of the social spectrum which I really enjoy doing,” she says.

“And if you can throw in a corset one week and not a corset the next, then that kind of makes it a little bit more interesting. I’m lucky because of all the work I’ve done with Mike Leigh, and all the opportunities he gave me throughout my career to play all these really vastly different characters, it’s kind of stuck. And it is what gets me up in the morning really. I don’t want to just be a one-trick pony. So although, frankly, you can leave the corsets ... I don’t like wearing them, but if somebody pays me enough, I’ll put one on.”

James Graham, who wrote the series and serves as executive producer on the show, was an eyewitness to these mysteries.

“I grew up in the community where the two murders that this story is inspired by took place,” he says. “I think it was remarkable that the overlapping symbolism with the first murder set into place with somebody using a crossbow and then as an outlaw, disappearing into Sherwood Forest. ... And I think narratives and mythology and folklore do feed into this story to expand it, I think, slightly outside the traditional crime drama.”