Yalitza Aparicio only auditioned for Roma because her sister was heavily pregnant and in no fit state to. Neither had acted before. But her older sister was desperate to know what the audition would be like, so she sent Aparicio along as a proxy. “I didn’t want to do the casting,” she says. “My sister pushed me because, in our community, they have never come before to ask us to be in films.”

Aparicio got through the first audition, then the second and, finally, months later, the third. Now there is talk of her winning an Oscar. If she does, it will be deserved. Alfonso Cuarón’s Roma is a masterpiece, and Aparicio’s performance is astonishing.

Roma is named after a district located in the Cuauhtémoc borough of Mexico City where it is set. The story, filmed in black and white, is simple – it is the early 1970s and a seemingly happy, upper-middle-class family implodes after a quiet betrayal. At the same time, Mexico explodes with student riots and an army massacre. Roma is the story of Cuarón’s childhood and it is entirely his movie – he has written, directed, edited, shot and produced the film. Yet, on screen, the film belongs to Aparicio, who plays the family’s nanny/maid, Cleo – everything is seen through her eyes.

Roma pulls off the impossible. It is both spine-tinglingly intimate (the texture of skin, the slop of water on paving stones, hushed conversations in the night) and thrillingly epic (the sea has never seemed so terrifying). It is a film in which nothing and everything happens.

Aparicio as Cleo in Roma. Photograph: Alfonso Cuarón/Netflix

When I meet 24-year-old Aparicio in London, I am surprised by how young and glamorous she is. As Cleo, she looks so world weary and browbeaten. Today, in a leather jacket, striped dress, heels and gold earrings, she is every inch a star. As she speaks Spanish, we chat through a translator.

Cuarón, who won the best director Oscar for his most recent film, Gravity, spent an age searching for his Cleo. At times, he despaired of finding her. He was determined to cast somebody who not only looked like Libo Rodriguez, the maid from his childhood to whom the film is dedicated, but had a similar nature, and understood the world she moved in. Cuarón sent out “armies of casting directors”, first to Mexico City, then to the states of Oaxaca and Veracruz. Eventually, his team came across Aparicio, in the southern town of Tlaxiaco, Oaxaca, the same state where Rodriguez grew up, and an eight-hour drive from Mexico City.

Aparicio had not heard of the film-maker. In fact, she was wary of attending the audition because she thought it might be a trafficking scam. When she discovered the director was Mexico’s most celebrated film-maker, it didn’t impress her. “It was only when I went to my final casting that I got to meet Alfonso, although it didn’t make any difference to me because I didn’t have a clue who he was or his role in the film industry.”

How did she feel when she was offered the part? “I was happy, but nervous because I had no idea whether I’d be able to achieve what I’d been asked to do by Alfonso. I was also a little sad.” Why? “I’d just got my degree and I hadn’t yet worked as a teacher. I’ve still not worked as a teacher.”

In the film, Cleo is adored by the family she works for, but ruthlessly exploited – forever at their service. In one of numerous memorable vignettes, the family sits down to watch a TV sitcom. One of the four children Cleo looks after lovingly puts her arm around her shoulders, and she reciprocates. As she relaxes for the first time that day, the mother of the house asks Cleo to get a cup of tea for her husband. A small detail, but one that says everything.

Aparicio’s Cleo displays an extraordinary empathy – with the children she nannies, their sometimes distraught mother or with her own lover, seen bizarrely demonstrating his martial arts skills in the nude. She is both childlike and prematurely aged; there is a lightness to her soul, yet her body is already stooped by the burden of work.

Yalitza Aparicio: ‘My life was similar. We were both poor, and we both wanted to go to Mexico City.’ Photograph: David Levene/The Guardian

We see her love in the tiniest gestures – a gentle back rub to wake up one of the kids, lying down in the sun to play dead with another, a human cushion for two of the children who fall asleep on her in the back of the car. Sometimes, the mother of the house treats her as a confidante (“We women are always alone in the end,” she tells her), and sometimes like dirt (“Goddammit! I told you to clean up the dog shit!”). Cleo is the heartbeat of the house and the film.

Aparicio saw many similarities between herself and Cleo. “My life was similar. We were both poor, and we both wanted to go to Mexico City to improve our family’s lives.” Her own mother worked as a nanny/cleaner, just as Cleo does. “She is still a domestic worker. When I was younger, I used to help her so she could finish earlier.”

She had always hoped that her teaching degree would provide an escape from poverty – for her and her mother. “I don’t want my mum to be a domestic worker any more because it is so exhausting.” She says she is not interested in indulging herself with the money she might make from the film. “My priority is to help my mum, and afterwards, we’ll see if I buy something for me.”

Mexico is such a macho society, she says, and, so often, women are left with the burden of responsibility. Aparicio still lives in a one-room home with her family. Her mother provided the inspiration for Cleo. “I wanted to be like my mum; as strong as her. She was my role model. The film is like a tribute to women in general – these invisible women are always there in the home, taking care of the children.” As a nanny, she says, her mother loves the children in her care with the same passion as she does her own children.

Where Roma is a paean to women – not only Cleo and her fellow maid Adela, but the middle-class family’s mother Sofia and grandmother Teresa – the two main men in the film are feckless moral cowards.

Again, Aparicio says this is a world she is familiar with. Her father left home when she was in her teens and her two youngest siblings were still toddlers, leaving her mother to bring up four children.

I ask her what she would say Roma is about to somebody who hasn’t seen it. “It is about life,” she says with a touching gravity. “In life, you have ups and downs, but you should never give up, you should always try to get ahead.”

That’s not much of a clue, I say. She smiles, and has another go. “I would say it’s about a group of women that are together to overcome things, to support each other, that are solving every problem that comes to their family and them individually, and that in the end it makes them stronger.” It is a beautiful summary. Like Cleo, Aparicio has a wisdom beyond her years.

Does she agree with Sophia’s statement in the film that women are always alone in the end? “Yes. Even when people speak up for you, they will never solve your problems so, in the end, you are alone. You have to stop thinking about what people are thinking or saying because, ultimately, that won’t provide you with support. In the end, you have to do it yourself, either for yourself or your children, and overcome everything.”

Roma is a throwback to the great Italian neorealist film-makers Roberto Rossellini and Vittorio De Sica. In films such as Rome, Open City and Bicycle Thieves, they cast non-actors alongside professionals to get more naturalistic performances – they wanted them to react more than act. He cast Aparicio’s best friend, Nancy Garcia, as Cleo’s fellow maid and roommate, Adela, to make their relationship more convincing and so Aparicio would feel more at ease. Cuarón also used a technique favoured by another master realist, Ken Loach – not giving the cast a script or telling them the full story, shooting in chronological order, feeding them lines bit by bit so each new scene is as fresh as possible.

In one scene in particular, the impact is devastating – Aparicio weeps uncontrollably as she reacts to a catastrophic event she had no idea lay ahead of her.

What does she think of the talk about her winning an Oscar? “That sounds flattering, but to be honest I’m grateful for everything I’ve been able to see, places I’ve visited, especially the positive reaction I have had for the film. That’s enough for me. My mum just tells me to enjoy everything.”

‘The film is like a tribute to women in general – these invisible women are always there in the home, taking care of the children.’ Photograph: Carlos Somonte/Netflix

I ask her if she regards herself primarily as an actor first or a teacher now. She thinks long and hard, before saying there are similarities. “As a teacher you set an example, you become a role model, and this experience of being in the film has made people in my community look at me as a possibility that their dreams could come true.”

The more time you spend with Aparicio, the more you realise how close she is to Cleo – gentle, sweet, with a sincerity that verges on the sombre. Would she like to make more films? “Yes, but I also fear it because I don’t know what it would be like to work with other directors. I believe each of them has their own way of working.” She is still thinking about whether she dare call herself an actor after only one film. “I don’t think I am an actor because I haven’t studied to be an actress,” she says, “but I don’t know what people would say.” Perhaps the fact that she hasn’t studied to be an actor is what makes her such a great one.

Roma is in select cinemas from 29 November and on Netflix from 14 December