'The Beast Must Die' star says 'nobody' saw the first season's twist ending coming: 'It was a complete mindf--k'

• Warning: Spoilers ahead for Monday's "The Beast Must Die" season finale.
• Star Cush Jumbo said it was "a complete mindf--k" to find out that an important character had died.
• "It's actually really hard to act when you know," Jumbo said about acting out plot twists.

Warning: Spoilers ahead for "The Beast Must Die" season finale, which aired Monday on AMC.

Cush Jumbo said "nobody saw the ending coming" for "The Beast Must Die," including her own friends and family.

In the six-part first season of the AMC thriller, Jumbo plays a distraught mother, Frances Cairnes, trying to find the reckless driver who killed her young son in a hit-and-run accident. Her search takes her to the home of the manipulative George Rafferty (played by "Mad Men" star Jared Harris). Meanwhile, Detective Nigel Strangeways (played by Billy Howle) is also trying to solve the case while dealing with PTSD from the death of his partner.

The fifth and penultimate episode ended with George (who is presumed to be responsible for Frances' son's death) dying unexpectedly under mysterious circumstances. The finale, which aired Monday, reveals that Frances was responsible for George's death.

Jumbo told Insider in June that it was "a complete mindf--k" when she first found out about George dying and explained why that murder scene was so tricky to film.

In the second-to-last episode of the season, Frances and George go on a boat ride. At the time, viewers (and George) don't realize that Frances is secretly poisoning George.

Monday's finale reveals that Frances had been killing George in the previous episode by poisoning some of the brandy they both drank when they were out sailing earlier that day. While she took a few sips of the pill-laced drink in that scene, George finished off the majority of the flask. It was enough to kill him, as Frances was hoping, and he died hours after the boat ride.

The "Good Fight" actress explained it was tricky to film that sequence on the boat in the fifth episode because while she already knew what was happening in it, the audience wasn't meant to find out she was murdering George in that scene until the finale, and she wanted to avoid accidentally giving that away too early.

"It's actually really hard to act when you know," Jumbo said about the plot twist.

Jumbo called George's mysterious death before the finale "one of the best twists" of the season, because his death raises more questions than it answers.

"It really throws you into thinking, 'What the hell is going to happen in the next step?'" Jumbo told Insider. "At the same time, I was secretly satisfied because you know, whether he killed her son or not, he's an a--hole." (The season never definitively answers the question of who killed Frances' son, but heavily implies George was the culprit.)

Harris, who plays George, also agrees that his villainous character is a "despicable person," telling Insider separately in June that he wanted there to be "something shameful" about George's death.

"I was pitching for [George] to be found like Elvis on the toilet or something," Harris told Insider, referring to Elvis Presley's death in a bathroom from cardiac arrest attributed to suspected drug use. "Where there's trousers around his ankles or something because he didn't deserve a dignified death."

Season 2 of 'The Beast Must Die' will focus more on Detective Strangeways' past

Jumbo confirmed to Insider in June that it's "highly unlikely" that Frances (who dies in the finale) will make an appearance in the second season of "The Beast Must Die."

Instead, Howle separately told Insider, the upcoming season will let the viewers in on more of his character Detective Strangeways' "history and what brought him to where he is today."

"I'm interested in his relationship with his line of work," Howle said about the troubled detective. Strangeways is the central character in a series of mystery novels by Nicholas Blake (the pen name of Cecil Day-Lewis, father of actor Daniel Day-Lewis), including the book of the same name upon which the first season of the AMC series is based.

Howle hopes the new season will explore more of his character's inner life and why he's chosen to become a detective.

"It takes a very specific sort of person to want to do that kind of thing. It's a very high-stress job. It's a very high-stress environment, but it's to do with a sort of sense of righteousness, a sense of justice," he said.