All Ella Purnell ever wanted to do was write children’s books about magical trees and talking ducks and happy bunny rabbits, and instead here she is chainsawing a man’s head off in a radioactive wasteland. Or freezing to death in the wilderness and being eaten by her closest friends. Or being so traumatised by school bullies that she takes up serial killing, slaughtering victims with her dead dad’s treasured pocketknife.

It might not be exactly what Purnell had in mind for her 20s, but spending the past few years committing, and being subject to, acts of stomach-churning violence has certainly had its upsides: the Londoner is now on the brink of TV superstardom. Purnell has been on a steep trajectory since 2021, when she appeared in hit US drama Yellowjackets as the prom queen captain of a New Jersey high school football team left stranded in a Canadian forest after a plane crash (think Mean Girls meets Lord of the Flies). In April, she starred in Amazon’s sensationally successful video game adaptation Fallout as Lucy MacLean, a vault dweller in a post-nuclear apocalypse United States who surfaces to search for her kidnapped father (the series attracted 65 million viewers in its first 16 days of release, and helped Purnell accumulate 1.4 million Instagram followers). Now, the 28-year-old is returning to the UK for more viscerally disturbing action. In new Sky Atlantic thriller Sweetpea, she plays Rhiannon Lewis, a receptionist whose mounting fury at being walked all over eventually erupts into a murderous spree.

Purnell isn’t sure why she’s ended up specialising in such troubling material. “People think I must be really messed up, but I swear I’m a happy, well-adjusted human!” Seated on a sofa in the corner of a blindingly white photography studio – the dramatic purple eye makeup from her shoot still intact – Purnell certainly seems psychologically sound; relaxed and genial, with a kind of preternatural confidence (even by American standards: a recent New York Magazine profile described her as “strikingly self-assured”). Yet the actor also joins me in identifying as a total wuss: “I can’t do horror films. I don’t really love watching too much gore, or any supernatural things. I’m not even really a big sci-fi person. And I’m not a gamer. So I don’t know how any of this has happened!”

In truth, there is a logic behind Purnell’s CV – and it can be traced back to the ambivalence with which she has approached her profession. Purnell “never really planned on becoming an actor. I feel a lot of guilt and impostor syndrome attached to that statement, because I know a lot of people have wanted to be an actor ever since they were kids. And that just wasn’t me.”

Confusingly though, Purnell actually was an actor when she was a kid. Growing up in east London, she took singing and dancing lessons at the storied Sylvia Young Theatre School, which led to her performing in Oliver! in the West End when she was 10. She appeared in her first film when she was 14 – playing Keira Knightley’s character Ruth as a child in Never Let Me Go. But as her career picked up further in her teens, she “freaked out” about the path that had been laid out for her (not by parents or agents, she clarifies, but a road she had unknowingly carved out herself). At 18, she decided to put the brakes on her acting career. “I wanted to go to university and be a writer – I wanted to write children’s books.”

Purnell won a place at university (she can’t remember exactly where or what she was going to study, but it was related to creative writing), but ended up deferring after landing a part in Miss Peregrine’s Home for Peculiar Children – working with Tim Burton was her childhood dream and she couldn’t bring herself to turn it down. She thought it would be a one-off, but more irresistible jobs cropped up, such as the lead role in a US TV show. In 2017, Purnell moved to New York to star in Sweetbitter – a series about a young woman who comes of age working in the city’s buzzy restaurant scene – and relinquished her university place. “I realised I didn’t need to go to university to be a writer – I could just write!”

Despite her fixation on writing – she hasn’t written a children’s book yet, but thinks she “probably will one day” – Purnell set off back down the acting avenue with renewed purpose. The fact she wasn’t desperate for success gave her the freedom to pursue only the most challenging, difficult and stimulating parts – an impulse that drew her to stories that deal in emotional and physical extremes. With Yellowjackets, she was enticed by the challenge of taking “this stereotypical mean girl and breaking her down. I wasn’t sure if I could do it in a way that felt tasteful and not tropey. And it scared me.”

Fallout also pushed Purnell well outside her comfort zone. Not only was she taking on some precious source material with a fervent built-in fanbase (the show is based on the beloved video game series), but Lucy herself would have been a challenge for any actor: you just try pretending to be a bouncy, privileged naïf raised in an underground vault modelled on 1950s America whose quest to find your father on the nuclear war-torn surface allies you with a 200-year-old ghoul hell-bent on selling your organs while decimating your entire moral code.

Yet Purnell aced the brief. In her masterful hands, Lucy’s journey from priggish innocent to imperious action hero wasn’t just entirely plausible, it was uplifting, too: seeing such a beautifully rendered bildungsroman unfold across a ravaged planet proved strangely life-affirming. The show – wryly funny, deeply horrifying, breathlessly entertaining and a smash hit by every metric – transformed her life. “There’s the day before Fallout comes out and no one knows who you are, to a week after Fallout comes out and suddenly people know your name. That’s a slightly disorienting experience.” Still, the actor views it as an overnight success 20 years in the making. “I feel like it became a marathon and not a sprint. It didn’t feel as shocking to me as when somebody goes viral overnight.”

She never worries about the hype going to her head. “I have an incredible support system who would never let me get too arrogant.” It includes “three younger brothers whose main mission is to humble me any opportunity they get”. Their primary focus is on “how short I am. And for context, I’m not that short. I’m 5ft 4in. But they’re all like 6ft.”

In the wake of this heady career high, Sweetpea might sound like an odd choice. The show, which airs on Sky this month, is unlikely to have quite the same reach as Fallout, which was the most watched show across all streaming platforms in April. It also marks Purnell’s return to the UK, having already established herself in the US big leagues. Technically she still lives in London, but she has worked so much in the States – and played so many Americans – that some colleagues have been shocked to learn she’s English. (“It’s a great compliment … Well it’s better than: ‘God, her accent’s so shit,’ isn’t it?”)

On the other hand, Sweetpea is the perfect step up. Purnell gets to be an executive producer for the first time, meaning she was involved in developing the series outline and the scripts, and also “got a large say in the visual elements. The hair and makeup is my favourite thing.” The actor – who remained glossily beautiful even in the gruesome Fallout finale – is practically unrecognisable as the downtrodden Rhiannon. It’s amazing what havoc a wispy fringe can wreak on a person’s appearance, but there were other details, too: “I’ve dyed my hair enough times to know what doesn’t look good, so having a mousy element to the hair that makes me look paler, highlighting the dark circles under my eyes, no makeup, being quite plain-looking.”

Reeking of awkwardness, loneliness and low self-worth, Rhiannon spends her life being either ignored or avoided. She’s a wannabe newspaper reporter who is frighteningly delusional about her crimes – yet her mission to fight back against a world that has mistreated her is surprisingly easy to get on board with. Although the series was designed as a prequel to CJ Skuse’s Sweetpea books, the Rhiannon in those is an out-and-out sociopath who derives huge satisfaction from killing, and is more “snarky and definitely a lot more evil”, says Purnell.

This TV version is clearly concerned with making her as sympathetic as possible. Purnell was compelled by the puzzle of creating a female killer viewers could connect with. “Women are so often pressured to be likable and pretty and perfectly packaged,” says Purnell, meaning it’s “a lot harder to create moral complexity”. She “empathised” with Rhiannon “but didn’t relate. I had to really do some mental gymnastics to understand [her behaviour].”

In the end, the team – which includes writer Kirstie Swain (Pure) and director Ella Jones (Back to Life, The Baby) – opted to focus on Rhiannon’s childhood trauma. We see repeated flashbacks to a school disco, where a bully called Julia snatches the wig Rhiannon wears to disguise the fact she’s pulled out most of her hair due to the stress of being tormented by Julia. When the latter, now an estate agent (played by Nicôle Lecky, best known for BBC Three drama Mood), is tasked with selling Rhiannon’s home, passed down to her and her sister by her late father, you can’t help but feel sorry for Rhiannon – despite her barbaric reaction. “When you go through an intense trauma, you get stuck in this arrested development,” says Purnell. She was determined to give Rhiannon a childhood bedroom untouched since adolescence (boyband posters, cuddly toys); she also petitioned for her to wear chunky black rubber-soled school shoes.

The actor clearly took immense care with Rhiannon: the result is a walking moral conundrum from whom it is difficult to look away – I don’t think I’ve ever had such conflicted feelings towards a fictional character. Once Rhiannon starts killing people, things begin to look up for her: she gets a promotion at work; the guy she likes stops dodging her. “She starts to feel more powerful and then obviously the more power you feel, the more power you exude – people read that energy. If you take away the murder, it’s quite a sweet coming-of-age story.” You can’t really take away the murder, though. The brutality and trauma that radiates through Sweetpea did “take a toll” on Purnell. She mitigated the effects through self-care – calling friends, watching TV, messing about with the crew – “because otherwise it could lead you down some dark paths. The job’s not worth your mental health.”

Evidently Purnell takes not being messed up extremely seriously, but some experiences on set left a lasting impression. When filming Rhiannon’s first murder, Purnell was told to maniacally stab a rubber dummy rigged up with blood bags for five solid minutes as the cameras rolled continuously. At first she didn’t want to – “It’s a little bit embarrassing” – but “after the first minute, I closed my eyes so I couldn’t see anybody, and just went in. I kind of blacked out.” Watching it back, she “had to look away because it was so uncomfortable to see myself in that state”. For the first time in her career, she had “lost 100% of my inhibition, going back to this very raw, animalistic version that probably does reside in all of us”.

By the end of Sweetpea, Rhiannon is in full flight-or-blood-spattered-fight mode: the show concludes with a scene so disturbing I can’t bear to rewatch it (and it’s one I am under strict instructions not to reveal). “Oh don’t, it’s awful. It does haunt me,” cries Purnell, throwing her hands over her face when I mention it (a move she repeats when I bring up a moment from Fallout that is also burned on to my retinas). She may still be reeling from all the things she’s done on screen, but it’s a price worth paying: when it comes to difficult – and disturbed – women, nobody is outperforming Ella Purnell.

Sweetpea is on Sky Atlantic, 10 October, 9pm.