What is interesting about what has been described as newcomer Lily Gladstone’s “big break” is that, in one sense, it is not big at all. This is the quietest film sensation imaginable – a role with minimal dialogue but major impact. Gladstone appears in Certain Women, written and directed by Kelly Reichardt, alongside box-office names Michelle Williams, Laura Dern and Kristen Stewart. Last October it won the best film award at the London film festival and Gladstone’s performance has been singled out. Rolling Stone’s critic, after dutifully saluting the stars (all three are excellent), writes: “Gladstone gives a performance of such piercing honesty and yearning, you almost can’t look at her.” The film, based on short stories by Maile Meloy, involves real women leading ordinarily complicated lives. It is nuanced, moving and thought-provoking and makes the Hollywood competition (even a film such as the Oscar-nominated Manchester By the Sea) seem forced by comparison. And while it is true you almost cannot look at Lily Gladstone when she is suffering, what this turns out to mean is the opposite: you cannot take your eyes off her.

It is a relief – as well as a pleasure – when I Skype Gladstone in Montana to find her talkative. Had she been like the character she plays, an interview might have been a tall order. She sums up the power of Reichardt’s direction early on, as “saying so much with so little”. It was shot in and around Livingston, Montana, on the Yellowstone river, surrounded by the Absaroka mountains, a town with a population of just over 7,000. Gladstone plays Jamie, a Native American working on a ranch – a temporary, winter job. She has only horses for company until she meets Kristen Stewart’s Beth, an unhappy law graduate who drives miles to give adult evening classes. Jamie sits in on Beth’s lessons not because of a burning interest in the law but because she needs a friend and because, although she does not talk about it, she is starting to fall in love.

As Jamie, Gladstone’s heart is visible in her wide-eyed, dignified, composed face. Her occasional and radiant smile surprises because it seems to have travelled such a distance from the seriousness that is her default position. We find ourselves as watchful of her as she is of others. Gladstone’s quality in the role is stillness – most striking when she sits opposite Beth in the local diner and listens. Gladstone tells me that stillness comes naturally to people from Montana. As the story progresses, there is never any confessional but she becomes ever more heart-rending.

Montana is a character in this film: beautiful, snowy and inconvenient. Kelly Reichardt recently explained its contribution: “There are mountains on all sides of you, which in many ways locks you in, and I think that changes the way you look at the world.” Gladstone refines the point: “Montanans are infused with landscape. You can sit in a landscape for hours and disappear. You can be part of the quiet. Or you hear the songs of the wind. I remember being hollowed out by the whistle of the wind as a kid. You were at the whim of your landscape and had to live with it.”

Gladstone was born in 1986 and grew up on the Blackfeet Reservation in north-western Montana, and, as her biography in the film’s press notes proudly elaborates, has “tribal affiliations that include Kainai, Amskapi Piikani and Nimi’ipuu First Nations”. She explains: “I lived in the reservations until I was 11, when we moved for lack of economic opportunities.” She describes her mother, an early childhood education specialist, as the “bedrock of our family”. Her father is “one of the most brilliant men I know – a gentle giant. He has deep spirituality and sharp intelligence.” In the 70s her father went to the University of Washington, but left early because it was “not an easy time for an outspoken Native American”. Later he went into broadcast journalism. Gladstone then adds a postscript about her forebears, revealing that her great, great grandfather, on her mother’s side, was first cousin to William Gladstone, the British prime minister, while on her father’s side, her great, great grandfather was Red Crow – a treaty chief (her family history would make a brilliant film in itself).

When her father moved the family to the suburbs, Gladstone had not anticipated how alien nuclear families would seem: “Everyone seemed so isolated in their little homes,” she recalls. What she missed was the “mutual reliance” of the reservations. She has kept in touch with the Native American communities in adulthood: “I am shaped by my community – and I have been supported in my ambition to be an actor and storyteller.’’

And how has it been to have the support of an even wider community – all these rave reviews? “It has, at times, been a bit overwhelming,” she admits. And was she ever thrown off track by acting alongside such a constellation of stars? “I was daunted after seeing the cast list and before auditioning. But my mantra for the rancher became: don’t think about it – one of the lines in the story.” And besides, she and Kristen Stewart (with whom she is mainly acting) clicked: “Kristen has an incredibly sharp, artistic mind, catches every micro-expression, understands how to drive a scene, gave me everything I needed. And we bonded over our admiration of Kelly.” When pressed to say what makes Reichardt special, she adds: “She gives her actors freedom and her world space. And she is good to be with – the best thing you can say about a person. She has a wonderful, dry, not overreaching sense of humour.” She volunteers that she also formed a bond with Michelle Williams because they both come from Montana and were even, she reveals with satisfaction, born in the same hospital.

She then tells the story of how Reichardt first went out to Montana and drove around until she saw a ranch she liked and knocked on its door. Lynn, the ranch owner, was resistant to the idea of a film at first. She said she did not want to exploit the beauty of her landscape for Hollywood. “But Kelly spends five minutes with somebody’s dog and…” Gladstone laughs – does not need to finish her sentence. Lynn and Reichardt quickly became friends but Lynn remained “hesitant”, especially about “having some actress she didn’t know working with her horses”. Gladstone had been around horses before but had no rancher experience – everything she learned was from Lynn. She says: “Lynn and I became close. She has no children and was born in 1953 – the same year as my own parents.”

When people praise the film for its “lived-in” quality, Gladstone believes they are reflecting her experience: “I lived on that ranch for two weeks. I got into the lull of daily chores – you have nothing but silence and rhythm. A lot of the character I found there.” To look the part, she even helped herself to Lynn’s boots and overalls with their ripped linings. And Lynn’s doubts were – slowly – put to rest. The horses came first, the film had to fit around them: “Reichardt is such an animal lover and would not let the horses go 15 minutes off their feed schedule.” Gladstone got to know the lead horse, Remington, by riding it every day: “I’m not as assertive as some lifetime cowboys but I figured it out.” It is charming to watch her offering Stewart’s Beth a lift on horseback to the diner and back – especially in a film where so much conversation happens in cars.

But what I kept wondering, as I watched the film, was this: how aware was Gladstone of what her face was expressing at every turn? “When I was little,” she laughs, “my father used to tell me he could see when I was lying because I’d get a twinkle in my eye. But I rarely make a meticulous choice in placing a gesture. I’m more fuelled by what is in the gut. One of the most intriguing things I talked about with Kristen is: how self-aware do you allow your characters to be? Sometimes, your audiences have an insight into the character that the character doesn’t.” The most important decision was about the “level of crush” her character had on Beth. “I told Kristen: ‘I’m not going to let Jamie be all that self-aware’ because, after all, who is?”

Lily Gladstone as Jamie in Certain Women. Photograph: IFC Films/Courtesy Everett /RE

Gladstone was herself coming to terms, during filming, with the end of a relationship. How much did the pain of that personal experience feed her performance? “It was a lesson in learning how to let go and walk away – not easy, but important wisdom. My relationship helped – although I’ve also often been in Beth’s position in my life.” And she briefly considers the difficulty of having a person besotted with you: “Beth’s hand is forced when Jamie shows up with that horse, there is nowhere she can go!”

Before Certain Women, Gladstone had been on the point of letting go in another sense: she was thinking of giving up acting. From 2009 to 2013, she had worked with Living Voices, a theatre company performing pieces in schools about “marginalised histories only touched on in textbooks”. And what I am wondering is: how hard is it to get work as a Native American? Gladstone admits that she has endured the predictable horrors of stereotyped casting: “After one audition, I was sent an email expressed in flowery, PC language, yet what they were really saying was: you don’t sound Indian enough. I tried to shame them by answering: if I’m not authentic enough for you, why not find a Native North Cheyenne speaker and have an English translation in subtitles at the bottom?” She was at a point where she felt: “I have done everything I can. I have to try something different.” And we agree that, sometimes, it is the act of letting go that creates the new chance. Tentatively, she suggests that the situation for Native American actors is improving. She is excited to be taking on the part of Isabella in a Native American Measure for Measure at the Oregon Shakespeare festival later this year.

When Gladstone first studied acting at the University of Montana (from which she graduated, with high honours, in 2008), in the opening semester, the teacher – who was introducing Theatre of the Oppressed – told her and her fellow students: “None of you is going to be famous. You are here because you want to study theatre.” Gladstone took to heart the advice that followed: “Keep yourself interested and invested in the world and you will stay an interesting performer. As soon as you disconnect from the human experience, you’ll be flat.” Her teacher then added: “And if any one of you does become famous, you can call me at four in the morning and laugh at me.” All I can say is that the teacher – who Gladstone does not name – would, from now on, do well to keep her phone on silent in the small hours.

Certain Women is out 3 March