Before Joanna Hogg's film The Souvenir premiered at the Sundance Film Festival in January, the name Honor Swinton Byrne had barely ever appeared in the press. The actress's breakout debut, then, was not only notable because it was her first-ever performance, but because Swinton Byrne, the 21-year-old daughter of Tilda Swinton and the artist-playwright John Byrne, had somehow avoided the fate of so many young scions today: Instagram fame and front-row ubiquity.

Instead, it was via a 24-year-old aspiring filmmaker named Julie that Swinton Byrne is making her remarkable first impression on the public. In The Souvenir, which is in theaters on Friday, Julie is a film student in London whose life is upended—first in a swoon, and later in jeopardy—by an encounter with Anthony (Tom Burke), a slightly older dandy who talks about art and the world as if they have both failed him. Of course, this kind of jaded, knowing world-weariness really works on a young girl from the country (even if Julie's family is well-off) navigating the big city in the 1980s, and so Julie is caught unawares later by Anthony's secret life of addiction.

Director Joanna Hogg, Honor Swinton Byrne, and Tilda Swinton photographed by Celeste Sloman for *W* magazine.

Julie is defined almost entirely by her arresting earnestness and naiveté; in the realm of bildungsromans, she's something of a stock heroine. (Even Julie is quick to describe herself as simply "average.") But Swinton Byrne captivates through a quiet self-assurance and natural dialogue that she improvised on set ("bringing to mind the mellow mettle of Vicky Krieps in Phantom Thread," wrote Richard Lawson in Vanity Fair). And Hogg, well aware what a star she's found, isn't about to let her go—Swinton Byrne has already signed on for Part II of The Souvenir, in which she'll star opposite Robert Pattinson.

Hogg had been toying with the idea of making a film about her life since 1988, two years after she first worked with Tilda Swinton, who plays Julie's mother in the film. They've known each other since they were 10, and were personally trained by Derek Jarman in how to use a Super 8 for Hogg's graduation project. (Julie's plans to make a film about life in Sunderland were actually Hogg's: "I start[ed] to conceive a film that would have been in black and white set in that very depressed part of the world," she said in an interview in 2016. "I was fascinated by the dying shipbuilding industry—after Thatcher got in in ’7, that part of the country was very depressed.")

"We’ve never worked together since then, though we've always been looking for opportunities," Swinton said last week. She had joined Hogg and her real-life and onscreen daughter in the sunlit offices of the indie film distributor A24, which snatched up the film and its sequel even before it took home the World Cinema Dramatic Grand Jury Prize at Sundance. "It’s just never been right until this one." She and Hogg had settled on her role as Julie's mother early, but two weeks before production began, they still didn't know who would play Julie.

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Swinton was asking around about her friends' children, and even friends of Honor's, for the role. And then it clicked. "Either my unconscious thought of someone and just stuck it in a cupboard, or I just was being thick...but finally, quite close to the wire, Joanna and I both looked at each other one day and went, 'You know, there is another person we haven’t thought about,'" Swinton recalled, turning to Honor with a smile. "The perfect person."

Joanna Hogg and Tilda Swinton, photographed by Celeste Sloman for *W* magazine.

Celeste Sloman

When Swinton and Hogg sat down with Honor and popped the question, she was surprised. But, Honor recalled, "I said yes straight away.... I was really sure that it was what I wanted to do. I was so excited and so delighted, but so, so surprised. I really, really didn’t see it coming," she said.

"It was a moment," Hogg added.

Swinton Byrne said that somehow the idea of being an actor hadn't crossed her mind before that moment. As earnestly and self-deprecatingly as Julie, she continued, "It never occurred to me that I would be remotely"—she paused for a long time—"remotely valuable to this at all."

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Tom Burke as Anthony and Honor Swinton Byrne as Julie in *The Souvenir,* 2019.

Sandro Kopp, courtesy of A24

Maybe it was Swinton Byrne's remote upbringing in the Scottish Highlands that made Julie so familiar to her. The Swinton clan has lived there since 876 A.D., as Tilda told W previously, and Honor and her twin brother Xavier were "up trees most of the time." They attended Drumduan, the grades- and test-free alternative school in the Highlands—a place where students make knives and build canoes—that Swinton cofounded in 2013.

Julie says during her film school entrance interview, "I feel as though I want to not live my whole life in this privileged part of the world I come from. I want to be really aware about what’s going on around me. I don't want to be in that bubble my entire life." And watching Julie's bubble—her nice life in her nice Knightsbridge flat, her nice groceries from Harrods across the street—be burst by Anthony, who takes advantage of all that is nice about her, can be painful to watch. But Hogg protested that revisiting her past—the cruel manipulation of a young woman—hasn't caused her pain, necessarily. "Sometimes I feel a bit of shame—of myself, and kind of how I was then, and how I reacted and how I got caught up in things." She paused. "But maybe there's a pain in the shame." She paused again, then let out a bitter laugh. "That’s terrible. Oh god."

Director Joanna Hogg and Honor Swinton Byrne photographed by Celeste Sloman for *W* magazine.

The early stages of writing, Hogg allowed, were tough. But filming itself was "a joyful experience"—at least for her. She's aware that she passed "the cloak" along to Swinton Byrne, who confirmed Hogg's suspicion that the process was a different experience entirely for her. "It was very, very, very difficult. Very, very difficult. I had a lot of empathy for her, and, I found, it was very hard to detach," Swinton Byrne said of playing Julie.

"I haven’t fully come out of it yet, because I can't—" she continued, before Swinton jumped in: "And you shouldn't, because we’ve got another one to shoot."

I asked Swinton, who has in the past scoffed at the idea that she would give her daughter acting advice, if she felt protective of Honor as she was put through what Julie is put through in the film, much of which Swinton had seen firsthand in Hogg's own life in her 20s. "Always, always protective," she replied immediately. "And always incredibly supportive, and particularly supportive at this certain age where it's important to have independent experiences and adventures—you know, to be the backstop. But to be there in full support is a great privilege."

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Tilda Swinton as Julie’s mother, Rosalind; Tom Burke as Anthony; and Honor Swinton Byrne as Julie in *The Souvenir* (2019).

Nicola Dove, courtesy of A24

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Tilda Swinton as Julie’s mother, Rosalind, in *The Souvenir* (2019).

Sandro Kopp, courtesy of A24

In a way, The Souvenir was the logical next step in the mother-daughter relationship; When they began filming, Honor had just graduated from Drumduan School in Scotland. They begin shooting the sequel this summer, and Honor is now using the time between films to prepare for her exams to get into university, where she's been thinking about studying psychology and neuroscience.

When Honor saw the completed first film, she felt "such a relief," she recalled. "I could step away a bit, and enjoy it from a little bit outside." She paused. "It didn’t really feel like a film at the time. It kind of felt like I was living that...and to know it was all filmed, to a certain extent, was a nice rest."

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Honor Swinton Byrne as Julie and Tilda Swinton as Julie’s mother, Rosalind, in *The Souvenir.*

Sandro Kopp, courtesy of A24

It's not that Swinton Byrne is rejecting acting or her sudden rise in profile. As was the case with acting, fame just simply hasn't occurred to her yet. Partly because of Hogg's orders to "stay in that zone, with blinkers on," and partly because she was "so unbelievably self-conscious" of her performance, Swinton Byrne is only vaguely aware of the reputation that is growing around her. She has yet to read anything about herself, and has abstained from delving into promoting the film almost entirely. Whether or not that's kept her in shape for, as Hogg put it, "the next part of the journey," it certainly seems to have helped with protecting her sanity. "I feel fine. I feel grown. I feel great," she said.

But she's also aware that, like Julie, she still has a bit of growing to do. Whether that will continue to be through acting is anyone’s guess. “I don't know. I don't know," she insisted, then offered a Julie-ism. "Life is an adventure," she said, with a laugh.

Related: Tilda Swinton and Her Daughter, Honor, Posing Together on the Red Carpet Is So Pure