Willa Fitzgerald tries to walk six miles every day. Based in New York, the 33-year-old actor, who’s known for Scream and Reacher, considers the outdoors stroll to be both a form of meditation and a way to prepare for her next role. “I do my best thinking when I’m walking,” she says. “I start exploring my character’s thoughts, and how they would be walking down the street instead.”

The irony is that Fitzgerald doesn’t seem to do much walking in her enjoyably nasty, knotty horror flick Strange Darling. Depicting a woman known only as The Lady, Fitzgerald is first seen frantically fleeing from a man who’s wielding a rifle; battered and bruised, she leaps through woodland, begging for salvation, until something utterly unexpected occurs. The film, then, demands a lot of Fitzgerald, and it’s understandable why she may have required lengthy hikes to get into the right headspace. “The character was informed by this desperate need,” says Fitzgerald over a video call, sat on her living room sofa. “She has wounds. She’s running. There’s a lot of things she’s encountering that are altering her body.”

Written and directed by JT Mollner, Strange Darling has arguably already earned cult status through its gnarly premise, colour-coded cinematography, and word-of-mouth acclaim. Deemed a “masterpiece” by Stephen King, the disturbing thriller has, for months, built up a strong reputation as a movie best watched cold. In fact, Fitzgerald insists at the start of our Zoom call, held the week of the film’s UK release, that we don’t reveal any spoilers. The trailer, too, is deliberately misleading. “It’s challenging,” she admits. “But we’re really trying to minimise the things we’re giving away.”

What can be disclosed about Strange Darling is that it examines serial killers, and what drives them romantically and emotionally. Told in a non-linear fashion, the film starts with a “chapter three” heading in which The Lady, as mentioned previously, must escape a man known as The Demon (played by Kyle Gallner, full of rage). The film then reveals an earlier chapter in which The Lady is confidently donning a red wig outside a motel like she’s the femme fatale in a sleazy Brian De Palma slasher. Embarking on a one-night stand with The Demon, The Lady eyes him up and asks: is he a serial killer? It isn’t immediately apparent that both versions of The Lady are Fitzgerald. It isn’t just the wig; the actor completely transforms her entire demeanour.

While I’m expecting to learn of Fitzgerald’s deep-dive into true-crime documentaries and anecdotes involving method acting, she explains that much more of her preparation involved watching animal videos, particularly raccoons. “The Lady is in a purely survival-driven situation,” says Fitzgerald. “What’s more animalistic than that? I spent a lot of time making a playlist for the character. It was a lot of exploration of her body in the world, and what her body is. That’s where my focus was, rather than in any real academic-y research.”

Born in Nashville, Fitzgerald wasn’t always planning on being an actor. Possibly in response to her parents being former rock musicians (“they lived a long and wild life before I ever came along”), she was keen to avoid the uncertainty of a career in the creative industry. “That’s why I went to Yale for psychology and not theatre,” she says. “I was in the psychology department for three years. At the same time, I was doing multiple plays per semester. I got agents without really looking for them. I got my first job before I moved to New York. I found myself on a path where acting felt undeniable.”

Fitzgerald’s mainstream breakthrough was in 2015 when she became the main lead for two seasons of the Scream TV series, an experience that taught her how to deal with high-profile exposure and last-minute script changes. Around this time, she also had her first big studio movie audition (she didn’t get 10 Cloverfield Lane but got as far as screen testing in LA). However, even with Strange Darling and a show literally called Scream, she jokes, “It’d be funny if I was like, ‘I’m a scream queen!’ It’s not a term I’d use to describe myself. But I love working in the genre world.”

While Strange Darling demonstrates Fitzgerald’s versatility, it was already apparent to anyone paying attention. Whether she was Meg March in the BBC’s adaptation of Little Women (“My walking habit started in Ireland, if we’re going to call it a habit”) or Roscoe in the pulpy action series Reacher (she confirms it’s “highly unlikely” she’ll return for future seasons), Fitzgerald could seemingly do it all. Recently, she wrapped on a medical drama for Netflix called Pulse that airs next year. However, on the movie side, Strange Darling is her announcement to the world as a bona fide lead, especially as the twists rely on the character’s inscrutability as she shifts personas before our very eyes.

Lensed on 35mm by Giovanni Ribisi in his debut as a cinematographer, Strange Darling is shot full of purpose, even if that purpose isn’t fully apparent until the final credits. We know about the male gaze and the female gaze. But what about the serial killer gaze? Fitzgerald is in practically every frame, and when she isn’t on screen, the camera is usually on someone out to kill her. “It was one of the most exhausting jobs I’ve ever had,” she says. “We were shooting 12-hour days because it was all daylight-dependent. But I left work every single day so tired and so excited to go back the next day, because there was this amazing sense of artistic collaboration. You don’t get that on every job.”

As Strange Darling is a film that revolves around twists, it plays entirely differently upon a second viewing. Likewise, Fitzgerald’s embodiment of The Lady is far more complex than cinemagoers will initially realise. “Every moment is actually incredibly sincere and functioning from a place of need,” says Fitzgerald. “The Lady is a character that has such depth without the audience necessarily understanding it when they meet her. That was something JT and I had lots of conversations about before I took the job. It was important that the story wasn’t mean-hearted, and that it actually had real depth, humanity, and love in it.”

It could be too early to know or perhaps she’s being humble, but Fitzgerald is hesitant to describe the impact Strange Darling has had on her career. Either way, Fitzgerald’s ascension in the movie business feels certain – JJ Abrams, in a rare tweet, called it “terrifying, hilarious, heartbreaking, sexy, and wild” and the film of the year – and even the few negative reviews of Strange Darling have acknowledged Fitzgerald’s star power. “Playing this character was incredibly risky, incredibly scary, and incredibly challenging,” says Fitzgerald. “It’s something that took my whole body and mind to pull off. It’s very special to do something you feel unequivocally proud of.”

Strange Darling is out in UK cinemas now.