Celine Song’s directorial debut, Past Lives, has been met with nothing short of adoration since its debut at Sundance. Indeed, since its release in the States, the film has received almost unanimously rapturous reviews for the subtle, yet richly layered, way it explores how love can span continents and decades.

Starring Greta Lee (The Morning Show, Russian Doll) as Nora, the story follows her family’s journey from South Korea to Canada and later to New York City, drawing parallels to Song’s own life experiences. Teo Yoo (Decision To Leave) plays Hae Sung, Nora’s childhood soulmate from Korea, while John Magaro (The Big Short, Carol) portrays Arthur, her husband in the United States. The movie delves into the relationships between Nora, Arthur, and Hae Sung, exploring themes of love, shared memories, and the Korean idea of inyeon – the red string of fate which connects people across different lifetimes.

Though Song was an acclaimed playwright and had worked as a writer on The Wheel Of Time, she didn’t have a lot of directorial work to showcase to producers, and was dependent on the strength of her screenplay to sell the film, which, on the surface is a simple love story. But it was important for Song that producers understood the depth and emotion of the film, explaining that, “It’s about the parts of yourself that we lose as we become the people we are, and the ways our lives are shaped by those we love.

“The pitch itself is a very simple story that might have happened to you or me or somebody you know,” Song continues. “It’s about an extraordinary moment in an ordinary person’s life. But I think those moments are so beautiful. The goal I have for the movie is to feel authentic to what it feels like to be a person. So the questions for me that were important were, ‘Is this going to feel real? Is it going to be something that I can believe in? Is this something I can trust and think that is what it’s like to be a person who is living now?’”

The film begins with Nora, Hae Sung and Arthur sitting together at a bar. Nora is between the two men as they both gaze at her, and she speaks first to one, then the other. The men never speak to each other. Suddenly, Nora looks straight into the camera, at us, the audience. She knows we’re watching. It’s a stunning opening that evokes the intimacy of being caught eavesdropping, and the moment was inspired by a moment from Song’s own life, when she, her husband and her childhood sweetheart all met in a bar.

“I was sitting there and I remember a very specific feeling, that the moment felt really extraordinary in some ways, but of course a version of the story happens every day in New York City or in Dublin,” she says. “I was sitting there and thinking about what’s going on between the three of us – for example, the two men don’t speak each other’s language. And they’re from different cultures, and they know me really differently, right? They know me completely differently, and they could never know what the other person knows.

“So I think there’s something about it that was really special. But I also noticed as I was watching other people in the bar, I could see they were looking at us trying to figure out who we were to each other, because we were kind of a weird trio. So I came up with the structure of the first scene, which is this scene in the bar, and when Nora turns to look at the look at the audience, it’s kind of an implication on the audience. But it’s also that you’re casting the audience to play detective for the mystery of this movie: Who are these people to each other? By the end of the movie, the audience will feel very connected to the answer.”

This opening scene plays with closeness and distance which are key themes in Past Lives. The film constantly addresses the ideas of liminal spaces, as Nora moves between places, languages and relationships where she feels like she belongs, where she feels like she doesn’t, and where she feels caught in-between worlds. These are ideas that Song is not just interested in due to her own immigrant experience and her artistic career, but in her interest in how the self changes over time.

“I think being immigrants or being an artist, you’re always going to be living in the middle space,” says Song. “But I also think liminality is something that we live in every day. Because every couple of years you look back at yourself two years ago, and you just don’t fully recognise the girl you were. You’re thinking, what happened? How did I get from that girl two years ago to this girl? Well, you were in a liminal space between those two girls. Any change involves a liminal space.”

The performances are incredibly layered and naturalistic, and though Song’s background is in theatre where rehearsal is the norm, she chose not to rehearse a lot with her actors in order to keep each moment feeling real and authentic.

“Theatre is about consistency,” Song notes. “It’s about showing up every night and doing the same show accurately and free and alive. But of course, you have to do it over and over and over again, sometimes twice a day, right? It’s holistic, the whole night is what the performance is, and great theatre actors are great the whole night. It’s not about moments – but film is. In great film acting, you do not need to have consistency or repetition.

“No, the only thing you need is to have the one take that is right. And this movie is about so many extraordinary hellos and extraordinary goodbyes where the emotion is so rich. And of course, the nature of hellos and goodbyes is that can’t actually do them twice. So you’re trying to capture the magic in a bottle, and setting the actors up not to rehearse endlessly, but to really understand what’s happening in that moment, and capturing it as authentically as possible, once.”

The characters of Hae Sung and Arthur, who don’t meet until late in the film, are both aware of the important role the other plays in Nora’s life. They’re also aware that the other knows her in a way they never will. Song deliberately kept some distance between the actors during production, to heighten this sense of distance.

“I kept the actors apart until we shot the scene where they meet each other for the first time,” she says. “So they were feeling a little bit of distance. And there were other things, like they never saw each other rehearse or work with Nora. So Nora was able to form a chemistry with each guy and they couldn’t see it for comparison.”

Song laughs mischievously.

“One thing I know was very intense for the actors,” she continues, “was that I asked Greta to explain to each actor what it’s been like to work with the other. So the guys were feeling a little bit of jealousy and a little competitive! When they finally meet on screen, what we’re seeing is these two different chemistries and these two different worlds crashing together – and we get to observe. And this is what the movie is really about, which is there’s a conception of a person that you have in your mind, this idea of the person – and then there’s the reality of the person in front of you. And we get to see the men navigate that together.”

As audiences will see when they watch this stunning film, it’s more complex, rich and human than a traditional, melodramatic, love triangle. Song urges audiences to look beyond that trope to more interesting possibilities.

“I know it’s so easy because there are the people involved, to think of it as a love triangle,” says the director. “But at the end of the day, it’s really about a woman who’s choosing herself, choosing her life. And what’s amazing is that she is also able to be loved by these two men, who love her for two different sides of her. I hope that’s what people leave the film thinking about.”

• Past Lives is in cinemas from September 8.