When Sarah Paulson reflects on her career, the 45-year-old actress neatly divides her filmography into two eras: "Before M.C." and "Post-M.C."

Those initials belong to Marcia Clark, the prosecutor Paulson portrayed with striking strength and vulnerability in Ryan Murphy's 2016 limited series "The People v. O.J. Simpson: American Crime Story." After two decades of hustling on stage and screen as a respected character actress, Paulson delivered a transcendent performance that yielded an Emmy, a Golden Globe and a Screen Actors Guild prize.

"In terms of the public response to me or awareness of me, it became very different," Paulson says during a recent video chat from her Los Angeles home.

But with a career-defining role comes the concern that the part could, well, remain career-defining. "You always worry," she says. "Will people say, 'She's good — but she'll never be as good as she was as Marcia Clark?' "

Paulson's "Post-M.C." era has included appearances in high-profile films such as "The Post," "Glass" and "Ocean's 8." She also has carried on as a malleable member of the recurring "American Horror Story" ensemble, playing a dozen characters — an all-powerful witch and conjoined twins, among them — over eight seasons of the Murphy-helmed anthology.

Long the co-lead or sturdy supporting player, Paulson is now the driving force behind a star vehicle of her own. In "Ratched," Murphy's Netflix series, Paulson cracks open the mind of psychiatric nurse Mildred Ratched, a character created by Ken Kesey for his 1962 novel, "One Flew Over the Cuckoo's Nest," and immortalized by Louise Fletcher's Oscar-winning performance in the 1975 film of the same name.

As Paulson processes her most daunting "Post-M.C." undertaking yet — top billing, the titular character, an iconic cinematic villain — she grimaces, mouths an exaggerated "wow" and buries her head in her shirt.

"It makes me start to sweat," she says through nervous laughter, cradling herself. "That really is what it is, isn't it?"

Murphy was still developing Evan Romansky's spec script for a Nurse Ratched origin story into a full-fledged series when Paulson got word of the project and asked her agent to inquire. But while Murphy saw his longtime collaborator as a natural fit, he asked Paulson to think long and hard about what the part would mean for her career.

Playing the lead role on a serialized show, potentially over the course of several seasons, was more of a commitment than the anthology work Murphy and Paulson had done together. Murphy also wanted her to take on executive producer responsibilities for the first time. And he felt the steely character was somewhat against type for Paulson.

"In many ways, this is the hardest part that she's ever had to play because she couldn't resort to the thing that she loves to do," Murphy says. "Sarah always says, 'Give me a black tooth and an accent, and I'm in heaven.' But I was interested in what happens if she is required to do something that she's never done, which is absolute feral stillness and lethalness."

Accepting and embracing the challenge, Paulson signed on.

"No one could deny the multifaceted, multidimensional aspects of this role," Paulson says, "and that there are not a ton of them just hanging off trees that are for women my age, that are this complicated and this nuanced and this rich."

Set in 1947, a decade before the events of "Cuckoo's Nest," the first season follows Ratched as she schemes her way onto the nursing staff of a top California psychiatric hospital, which has been tasked with evaluating a notorious priest killer (Finn Wittrock). The series cites both Kesey's novel and Milos Forman's film as source material, though before "Ratched," the character's only canonical backstory was that she served as a nurse in World War II.

Although Paulson only read passages of the book, fearing that too much research would undercut her own instincts, she did want to revisit the "Cuckoo's Nest" film and form her own diagnosis for the source of Ratched's cruelty.

"I mean, there is no way to improve upon perfection," Paulson says. "Louise Fletcher's performance is so extraordinary that I thought, 'Oh, this might be really scary.' When I was younger, I found her terrifying. Of course, on second viewing ... I had to find a way that I could get inside it."

In 2003, the American Film Institute ranked Ratched as the No. 5 villain in cinematic history. Yet Paulson saw humanity in the film's version of the character — whose gaslighting combats any flames of rebellion at an all-male psychiatric ward in Oregon — calling her a "real victim of a patriarchal system."

Steadying the ship during production was Paulson, who, having made her directorial debut on a 2018 episode of "American Horror Story: Apocalypse," was asked by Murphy to be a full creative partner in the endeavor. As an executive producer on "Ratched," Paulson helped curate directors for the season's eight episodes, worked with castmates to dissect scenes she wasn't in, and made decisions on prop, set and costume choices.

"She's an incredible leader as a producer," says Sharon Stone, who is part of a "Ratched" supporting cast that also features Cynthia Nixon, Judy Davis and Jon Jon Briones. "To be able to watch a woman that I respect and admire have these opportunities is so heartwarming and so emotionally satisfying that I could burst into tears telling you how proud and excited I am for her. She's a very kind, very gentle, very reasonable, lovely, talented person."