“I was looking for something that would stretch me, and Gaby Chiappe’s script came across my desk and it was truly the best script that had passed over my lap, maybe ever,” she said. “I just couldn’t put it down, and I thought, ‘I have to do this’, so from the very beginning I was quite excited by it. So, yes, I was working hard, but I was enjoying it because I enjoy working hard.” In the limited series, Jumbo stars as Frances Cairnes, who takes matters into her own hands after learning the police have dropped their investigation into her son’s fatal hit-and-run. Posing as a novelist researching a new murder-mystery, Frances ingratiates herself with the family of George Rattery (Jared Harris), the man she suspects is responsible, and sets a plan in action to kill him.

She’s tracked by PTSD-stricken detective Nigel Strangeways (Billy Howle), who has reason to believe his department covered up Rattery’s involvement in the incident. Strangeways throws himself into pursuing justice for Martie, but when he learns that an “undercover” Frances is living with George and his family, Strangeways has to work overtime to both prove Rattery is guilty and prevent Frances from carrying out her plans for revenge. Frances is a character that’s been driven to the edge. The opening moments make that very clear, when she glares into the camera and says, “I am going to kill a man.” Next, she’s changing her hairstyle and moving into a new apartment. As the mother of a toddler, Jumbo could readily identify with Frances’ passion for justice. The series explores how a vigilante pursuit transforms her radically. “She’s got nothing, and for somebody who’s lost everything and doesn’t have anything to live for, if this motivation is the only thing getting them out of bed every day, it makes them keep going,” the actress said. “So it’s like playing this game of, sometimes she’s convinced of who the guilty person [is] and then other times, she’s really in doubt — not just of who it might be, but also of what she’s doing and whether that’s the right thing to be doing.”

There were different layers of Frances that Jumbo had to track. Not a method actor, she relied on emotional graphs, a rigid process which she mostly employs for stage work. It came in handy in becoming Frances, which was intense, as the script really dives into detail in its depiction of her grief. “If you spend 10 to 12 hours a day on set crying, or in deep grief, or being wracked with stuff, or doing something that’s physically grief-stricken — like I was in every day — then by Saturday you’re pretty depressed,” she said. “So imagine having to do that over months, that’s exhausting and you’ve got to pull yourself out of it eventually, which I know how to do.” Her 3-year-old toddler helped her pull herself out of it and stay sane.

Jumbo also had to train physically for certain situations into which Frances puts herself as she pursues George. Despite having played tough lawyers and detectives, Jumbo said all of these roles are stretched for her. “People think of me as quite a tough person who likes to take dark material,” she said. “Actually, I’m kind of a wuss.” There’s plenty to like about the series, but its strength is easily its cast, as Jumbo and co-star Jared Harris enliven their cat-and-mouse game throughout. Jumbo couldn’t have asked for a better colleague in the veteran actor, who had been attached to the project for a few years, but it just took the right combination of producing partners to come together and make magic happen. “My experience with Jared was unlike some male actors who are more experienced than you in terms of being positive in scene support, but you kind of get used to that as a woman,” said Jumbo. “It was a really heavy, emotional role to carry. And I’m not a method actor, I use substitution, but he was so supportive in every way.” Based on the novel by Nicholas Blake (pseudonym of Poet Laureate Cecil Day-Lewis CBE and Daniel Day-Lewis’ father) and shot on the stunning Isle of Wight, additional cast members for “The Beast Must Die” include Geraldine James, Nathaniel Parker, Maeve Dermody, Douggie McMeekin, Mia Tomlinson, and Barney Sayburn. Having left, like many Black British actors, for the U.S. in search of better roles, Jumbo is pleased to be back in the U.K., working. The role that had kept her in America and made her famous was that of sharp-witted lawyer Lucca Quinn in “The Good Wife” and its sequel, “The Good Fight.” It came about after her solo theatre show “Josephine and I” transferred to New York in 2015 and was seen by series star Christine ­Baranski, who loved it. It was quite a showcase: Jumbo wrote 23 characters for herself in the story of jazz singer Josephine Baker. “It means that, actually, for the first time, I feel like the U.K. is being slightly more accurately presented in its contemporary work, which is [new], especially for a U.S. audience,” she said. “The reputation that it had before was Cockneys and the Queen, and that’s kind of all that happens here. So it’s quite cool being able to open up the U.K. to the world in a more realistic way.” She doesn’t plan to return to Lucca Quinn in “The Good Fight,” and she has no qualms about leaving the character she has played the longest. “Over time your arcs have nowhere to go — it’s just the nature of serial television,” she said. “Some people stay for the check, but that’s not me. I have to be fulfilled.” Jumbo is currently filming Netflix’s Harlan Coben thriller “Stay Close,” and she’s also preparing to play Hamlet at the Young Vic, which kicks off its run in September. “The Beast Must Die” premieres Monday, July 12 at 10 p.m. ET on AMC.

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