There are just so many films to wade through at Sundance, from big-star vehicles to news-making documentaries, making it inevitable that smaller, less obviously worthy gems will fall through the cracks. There’s not been much noise around Animals, the second narrative feature from the Australian director Sophie Hyde, and in a year when the festival has achieved record-breaking diversity in its lineup, there really should be. For Hyde has forged a wonderful, utterly lived-in film about two women at a crossroads, one that attendees should be breathlessly, excitedly discussing around town, urging everyone else to see immediately.

There’s a far more conventional film to be made from what could be an overly familiar log-line: two best friends face conflict when one reveals she’s getting married. But what’s so startling about Animals is its undying ability to upend expectations. It’s not exactly packed with twists, but dialogue remains spiky and unpredictable, scenes don’t play out in the ways in which we’re used to, and characterisation is free of judgment and cliche. It’s easy to underestimate the unstructured delicacy of the script, written by Emma Jane Unsworth based on her book of the same name. There’s rich detail and truth underpinning her gloriously untamed study of friendship, one that refuses to play by the rules.

Laura (Holliday Grainger) and Tyler (Alia Shawkat) have been best friends for 10 years and they share an intense co-dependency, living together and spending their nights drinking, snorting and fucking their way through Dublin. While Tyler appears content to be a barista, Laura craves more but struggles to focus on her dream to be a writer, too easily distracted by her vices. When she meets pianist Jim (Fra Fee), her friendship with Tyler is forced to mutate, with inspiration and romance pulling her away from their life together.

For the first 10 minutes or so, Animals is a tad too deliberate, montaging a friendship that feels performed and Shawkat’s eccentric American is rather annoyingly outsized. But, as in Sarah Polley’s acutely observed marital drama Take this Waltz, we’re being inducted into a private world with two people who have a learned shorthand, one that might be off-putting but one that isn’t being neutered or softened for the viewer. It takes us time to acclimatise, to catch up on a decade’s worth of friendship, and Unsworth avoids an ungainly dump of exposition, details revealed with patience. It’s so rare to see a friendship constructed with such nuance and care, and there’s never a desperate plea for our investment or for us to even like the pair. We’re just seeing them as they are, messy and jagged.

Shawkat, the more tested comic performer, is reliably strong but surprisingly not the film’s standout. Instead, all attention is drawn to Grainger, an actor whose filmography has been marked by smaller, often thankless roles. She’s absolutely mesmeric here, one of those rare, special performances that feels like we’re under a spell. There’s such remarkable subtlety in her facial reactions, from an exquisite scene where she watches her new boyfriend play the piano – oscillating between excitement, intimidation and arousal – to familiarly uncomfortable scenes of her trying to write, her unsaid lack of self-belief threatening to derail any progress. It’s just so captivating to see her on a journey, one that eschews obvious signposting and flagrancy, the film never selling her out.

There’s specificity and profundity in how the film portrays the negotiation of a new relationship, figuring out how to maintain one’s own identity when romance blossoms and how to grow up without giving in. A lazier film would patronise and admonish the pair for the extremity of their lives, but there’s a strict resistance to any clumsy moralising. It’s an often funny but often painful watch, especially in one protracted party scene and the horrifying morning after, and the film is willing to venture into some audaciously dark places. Hyde’s direction is stylised but never suffocating, and she’s given her film a crisp, distinctive aesthetic without detracting from its reality.

Echoes of Trainwreck, Bridesmaids and Fleabag only serve to remind us of how unconventional and defiant Animals is in comparison. Its wild nature won’t be to everyone’s taste, but that’s sort of the point. It’s not a film that cares if you find these women charming or likable – it just cares that you believe them.

• Animals is showing at the Sundance film festival and is still waiting for a distributor