When sibling film-makers Josh and Benny Safdie were developing the script for Uncut Gems, their new film set in the shadier recesses of the New York jewellery trade, they scoured their native city for inspiration. For their protagonist, Howard Ratner, a risk-craving gemstone dealer played by Adam Sandler, they drew on stories their father used to tell from his time working in Manhattan’s Diamond District.

For Ratner’s girlfriend and partner-in-crime, Julia, the Safdies took a more direct approach: they based the character on a young New York socialite named Julia Fox, whom they met in the early 2010s. A glamorous fixture of the downtown club scene, Fox was running a minimalist fashion label and posting revealing videos of herself on the social media platform Vine. So closely did the Safdies mirror her in the character of Julia that, when they sent Fox a script a year later asking her to play the part, her first question was: “Have you been spying on me?”

“There were a lot of similarities,” says Fox down the line from Los Angeles. “Even in the character description, I was kind of like: ‘This is a little familiar.’ It was pretty spot-on.”

At the time, the film kept getting pushed back. But the Safdies’ profile was raised by 2014’s harrowing Heaven Knows What, and then went sky-high after they released the thriller Good Time, starring Robert Pattinson, in 2017 (the New Yorker called it “an instant crime classic in the age of Trump”). As as result, when Uncut Gems finally came around, Fox’s claim on the role had become tentative. The fact that she had never acted before didn’t bother the Safdies, who enjoy casting first-timers in their films. But this production was much bigger than before, with heavyweight producers on board including Martin Scorsese and Scott Rudin, and the brothers were under pressure to find a bigger name.

“They auditioned about 300 girls,” says Fox. “I heard Lady Gaga’s name thrown around. I heard Jennifer Lawrence and Scarlett Johansson. I think at one point I even heard Kim Kardashian, though I never asked because I didn’t want to put myself in an uncomfortable position where I would be obsessing over it and doing the compare-and-contrast thing.”

But she was determined to make the role her own. Not all aspects of the character were identical – Fox had never worked as a Diamond District shop assistant, nor had she used her seductive powers to get famous musicians to buy jewellery – but the parallels were clear. “Being independent, resilient, being a hustler, having a ride-or-die mentality, and overall just being really cute”: these qualities, says Fox, were shared by both Julias.

This unabashed way of talking about herself, verging on brash, seems characteristic of Fox, who later in our conversation declares that, at 28, she has outgrown New York, which she describes as a “small town”. But she can be sweetly self-doubting too, confessing to nerves on set and berating herself for playing it safe when she made her chat-show debut on Jimmy Kimmel Live! last month.

“I’m kind of an extroverted introvert,” she says. “I do really well in social situations, but I have a tendency to isolate. I think it’s what makes me an interesting person.”

It’s what attracted the Safdies to her, this mix of assertiveness and vulnerability. To convince the studios of Fox’s rightness for the role, they got her to do a scene with Sandler at a New York department store – “We were arguing over a dress he was buying me: I wanted the more expensive, better dress and he wanted the cheaper, not-as-nice dress” – and a karaoke scene where she sang Wicked Game by Chris Isaak. “I was really nervous about that,” she says. “Singing in public is already kind of nerve-racking. But I do love karaoke and Josh thought I would really shine.”

She secured the part, though it took her a while to settle. “I was nervous the first day because I was like: what if I tricked all these people into thinking I can do this, and then they yell ‘action!’, and I have no idea how to act?” she says. “I only slept one hour the night before, but it kind of helped, because in the scene Julia hasn’t slept – she went out partying and didn’t come to work on time – and everyone loved it. After that I felt so much better.”

Some early viewers of Uncut Gems, which follows Sandler’s character through a series of increasingly high-stakes gambles, have compared it to a cocaine rush; one critic for Variety described the film as “a protracted heart attack”. Given the Safdies’ penchant for shooting on the hoof, without permits where possible, it would figure if the production had been stressful too, but according to Fox it was easy and fun. “I just felt like I was making a movie with my friends.”

She enjoyed working with Canadian R&B megastar the Weeknd, with whom she shares an intimate scene in a nightclub, and retired basketball giant Kevin Garnett, whose obsession with an Ethiopian black opal sends Howard, and the film, into overdrive. “We were both first -time actors, so we had that bond,” Fox says of Garnett. “I think we’re going to be lifelong friends.”

It helped that Sandler, playing against type – ie not playing a man-boy in a goofy comedy – seemed a little anxious too. “So we were both nervous,” she laughs. “He just really lightened the mood and made the experience so fun and not intimidating. He has a very warm, welcoming energy.”

Some viewers have wondered how anyone could put up with the manic, spectacularly irresponsible lowlife that Sandler portrays in the film, but Fox can understand what Julia sees in Howard Ratner. “They’re both broken people who feel alone in the world. When you’re addicted to something like gambling and you’re always ripping off everyone and everyone is just an opportunity, it makes you feel kind of alone. But with Julia, he doesn’t feel that way – they’re both able to be vulnerable with each other. And he’s charming and manipulative and so is she.”

Julia Fox in Uncut Gems.

Has she ever met anyone like Howard before, or indeed dated anyone like him?

She splutters. “I would never date anyone like that. That’s also where the character and I differ. But I’ve definitely met people like that – there’s so many people like that in New York.”

With her languid drawl punctuated with kind-of-likes and you-knows and her worldly, seen-it-all-before attitude, Fox comes across as a consummate New Yorker, a Manhattanite to the core. In fact, she was born in Italy, spending her first few years with her Italian mother in the town of Saronno outside Milan. “I lived mostly with my grandpa,” she says. “He took care of us while my mom was in university getting her degree.”

Moving to Manhattan aged five to live with her American father was a culture shock, she says, though she soon found her feet. Her dad was extremely laissez-faire – “he always treated me as an adult, even as a child,” she recalls – and as a teenager Fox ran wild. “Very, very wild,” she says. “I wouldn’t come home for days. And I was allowed to do that. I think my dad was relieved that I was out of the house.”

What was she getting up to?

“Hanging out at friends’ houses,” she says. “And then we would go sneak into clubs – we all had fake IDs. It was New York, so you hung out in staircases and rooftops.”

Aged 17, Fox overdosed and says she had a near-death experience – she saw a tunnel of light and felt her spirit lifting out of her body. “It was after a night out and I really blacked out,” she says. “I don’t know exactly what happened but I remember afterwards just feeling so different, like something changed.”

Something inside her had changed?

“Yeah, I became like a straight-A student,” she says. “It was very spiritual, like an awakening. Being saved. It really felt like a divine intervention.”

Did the partying tail off a bit after that?

“Totally, yeah.”

Work filled the gap. When asked what she gets up to for fun, Fox says she loves going to the movies, writing and spending time with friends, “and running errands. I’m really big on running errands.”

Running errands?

“Yeah, getting things done. I’m just a doer. I like to stay busy. I think boredom is my number one enemy. I always need to be stimulated or feel like I’m accomplishing something, whether it be small or big. I’m a big list girl. If I don’t have a list, I’m totally lost. ”

In life, I don’t play it safe. I put everything out there. I’ve always done whatever I wanted to do

She’s also restless. After dropping out of a media studies degree at the New School university in New York, and then tiring of the fashion world, Fox dabbled in art and photography, putting on a 2017 show entitled RIP Julia Fox, featuring images of mutilated women drawn on silk using her own blood. It was partly inspired by her near-death experience, she said at the time, though Fox’s interest in bodily extremes goes further back. When she was 18 and not yet out of high school, she got a job as a dominatrix in a dungeon in the East Village. (There was no nudity or penetration involved, she made clear in a recent interview, just role-play.)

“That’s a ballsy thing to do as a teenager,” she says. “I was the youngest person there by far. It was nice, because I was kind of like the baby and everyone took care of me. It felt like I had a family unit, with a bunch of sisters. I’d only ever really been around boys or men, having grown up with my dad and my brother, so it was nice to have that female energy that had been lacking in my life. And I’ve been a girl’s girl ever since. I don’t think I have one guy friend who’s not gay.”

Considering her rich portfolio of jobs and life experiences, Fox feels frustrated that her “real personality didn’t really shine through” on her slightly awkward, tongue-tied appearance on the Jimmy Kimmel show.

So what did we not see?

“In life, I don’t play it safe,” she says. “I always put everything out there, and I’m shameless. I’ve always done whatever I wanted to do, and I think that intimidates people – whether it was publishing my very personal art book, or creating a fashion line, or even making this movie having never acted before and not knowing if I would be good at it or how it would be received.”

Fox at a Moschino show in New York, December 2019. Photograph: Lexie Moreland/WWD/REX/Shutterstock

As it turns out, Fox is very good in the film, radiating brassy, sexy attitude even as the cracks in Julia’s tough facade begin to show. (The performance earned her a breakthrough actor nomination at the Gotham awards in October.) The Safdies gave her plenty of creative licence to make the character properly her own, she says. “They really wanted it to feel real and believable, so everyone was allowed to freestyle a bit.” One scene, in which Julia reveals to a crisis-stricken Howard that she’s had his name tattooed on her back, was suggested by Fox, who had a similar experience with a boyfriend, whose name she had tattooed on her ring finger after a blow-up to show her commitment.

The appetite for creating dramatic narratives, as well as acting in them, has been building inside Fox for some time. Last year, before her role in Uncut Gems was confirmed, she went to Reno, Nevada, to direct a short film with a friend. They didn’t go with a firm plan in mind. Instead, she says, “We knew that there was a story there and we were going to uncover it. And then, immediately upon entering the city, we met all these kids on the side of the highway and I just knew that this was going to be where my story was.”

Speaking to the children’s parents, she learned that Reno was rife with drug use, crime and sex work. “There are girls going missing all the time,” she says. In the film, entitled Fantasy Girls, four teenage girls risk their lives to attend a talent show while a kidnapper is on the loose. Shooting it was a revelatory experience. “I realised, this is what I wanted to do for the rest of my life. I finally found it after trying my hand at so many different things.”

Right now, though, Fox wants to capitalise on the buzz around Uncut Gems, which has intensified since its premiere at the Telluride film festival last August. The Wall Street Journal said of it: “Everything is simultaneously terrifying, horrifying and mesmerising ... Uncut Gems is a dark sparkler.” (It may be too intense to get Oscar recognition, says Fox, but she hopes the Academy will keep an open mind.) In November, she signed with the talent agency WME, which will mean leaving “small town” New York for the wide-open expanses of Hollywood. “Working as an actress, I’ll be able to learn more about the film-making process,” she says. “So the immediate future is acting, but my long term is definitely writing, directing and producing.”

Destiny, she believes, has had a hand in getting her to this point. “Even if things didn’t go how I had envisioned them, but went another way, that other way was how they were supposed to go,” she says. The route – from city rooftops to East Village dungeons, fashion shows to garish jewellery shops – may have been circuitous and occasionally hazardous, but it’s all part of a bigger plan. “The universe has a funny way of making sense at the end of the day.”

Uncut Gems is in selected UK cinemas now and on Netflix from 31 January.