At 11am on a Friday morning, Studio City in Los Angeles is full of mums. Mums coming out of nail salons, mums dropping dogs off at day care, mums picking up fresh bread. Outside one deli, a local mum with cropped hair slides in through the front door in a camel-coloured overcoat. She moves towards a back booth and, complaining of a slight cold, orders chicken noodle soup. She could be any mum, but she’s the mum: Annette Bening.

She looks around the 1950s diner. “I love the booths, I love the atmosphere, and…” she smiles, pointing at portraits of stacks of salt beef and rye, “I love the pictures of the sandwiches.” It’s a surprise, perhaps, that such Hollywood royalty can sit at a booth in an unassuming deli without anyone bothering her.

For almost 30 years, Bening – a four-time Oscar nominee, a Bafta winner for American Beauty and twice a Golden Globe recipient (for Being Julia and The Kids Are All Right) – has been bending ideas of family. At the age of 58, her latest maternal portrayal in 20th Century Women is lauded as her most electric, searching and emotionally expansive yet.

Bath time: 20th Century Women trailer Guardian

“It’s a familiar dynamic to me – playing people’s mums,” she says. The strength of her leading ladies could have something to do with the amount of practice she’s had in real life. Despite her celebrity status, behind closed doors Bening has kept it traditional, even while married to Hollywood ladykiller Warren Beatty. The pair have four children together, the youngest born when Bening was 42. Their relationship began on the set of Bugsy in 1991. “That changed my life,” she says. They had a child the following year. The story goes that Beatty promised Bening he wouldn’t ask her out until they’d wrapped shooting. “That’s not true at all!” she says. “This seems to be a new thing of his, but it’s so not what happened. We were both single and it wasn’t like we were doing anything wrong. Though we did hide it.”

‘Bening has a rebellious streak in her, she’s subversive. She’s not going to do what you think she’s going to do’

They still do. Contrary to their blockbuster status, the couple are private. You won’t find Bening on social media. “No! Nu-huh,” she says. “That’s easier for me. But I feel for other people who have to be on there.” Privacy acts as a safeguard against certain conversations. A self-identified feminist, Bening argues that women are never assessed for their work alone. “Women in showbusiness have to deal with people talking to us about our children. Men generally don’t.” She doesn’t feel a sense of having compromised her career for family life. When Beatty and Bening did Love Affair together, they were trying for their second child.

“That I remember. You always remember when you get pregnant,” she laughs. “If I turned anything down, I was either having a baby or had just had a baby,” she says, flatly. They have managed to deflect the media’s attention so much that I’m surprised to learn that their eldest – Stephen Ira – is transgender. When asked about how parents deal with transitioning children, Bening grows fidgety, her eyes chastising me further as the words come out my mouth. For a moment I feel out of order. I would never ask the mother of a cis child such an invasive question about parental concerns.

“I’m super proud of him,” she says. “I leave it to him to say what he wants about his life. I don’t think it’s fair for me to speak for any of my kids, quite frankly. Not that I wouldn’t like to, ha!” she laughs.

Leading the charge: with husband Warren Beatty in Bugsy. The couple met on the film set Photograph: Tri-Star/Rex/Shutterstock

20th century Women, a film based on director Mike Mills’s mother, features Bening as Dorothea, a woman subcontracting out childcare so she could constantly work on their dilapidated family home to no apparent end. The movie suggests there’s a different kind of maternal streak in all of us, and that sometimes the mother becomes the mothered.

The film has been called the most feminist movie ever written by a man. It lives on Bening’s central performance.

“I’m just beginning to stop thinking about acting really,” she says. “I’m starting to let go, and that’s very freeing. I threw all preconceived notions out and wanted to be sort of empty, going from moment to moment not knowing what I was going to do.”

Thinking of American Beauty and Being Julia, I wonder if she finds it strange to work so often with male writers and directors on female-led stories. “Huh,” she says. “Well, look at Ibsen, the father of modern drama. The feminists at the time wanted him to be their posterboy and he refused. He called himself a humanist, not a feminist. That’s always stuck with me.”

Three’s a crowd: with Anjelica Huston and John Cusack in The Grifters. Photograph: Cineplex/Odeon Films

She praises Mills for investigating not just his mother, but all the women in his life. “He was interested in trying to understand and empathise with women. He has a very strong, interesting wife [Miranda July].” The answer, she says, is not just in women writing female roles, “but it’s also in women writing for men, in women empathising with people from all kinds of different backgrounds that aren’t their own.” It’s not enough for women to simply write the women they know.

Mills describes Bening as having the vision and intensity of an athlete. “It’s a trip how alive and subtle her performance is,” he says. “She’s like a ball bouncing down a hill, not predetermining anything. When you’re interfacing with her, there’s this very generous, genuine soul. It’s only when she’s not around that you think: ‘Holy crap, what am I doing with Annette Bening?’”

He attributes her unique acting style to her own experience. “She’s had a big, rich life as a woman and a mum, so she comes to her character with such emotional intelligence. I saw this consistent rebellious streak in her – this noncompliant, subversive soul. She’s not going to do what you think she’s going to do.”

Coming up roses: with Kevin Spacey in American Beauty. Photograph: Allstar/Dreamworks

The experience of Bening as a mother was an equally eye-opening experience for her co-star Lucas Jade Zumann, who plays 15-year-old Jamie. “He said I’d always give him life advice on the set,” she laughs, embarrassed. “Shit – that’s who I am, a person who gives kids unasked-for advice.”

Folding neatly into the film’s themes, Bening herself reckons she learned more from her young co-stars Zumann and Elle Fanning than they did from her. “They’re doing what they’re doing instinctively. That’s what we all want to find as grown-ups – simplicity.”

We didn’t make the film thinking Trump would win, but the way we see everything now is through a new lens

During a scene when Dorothea walks in on Jamie listening to punk band the Raincoats and says “Can’t things just be pretty?” the writing captures a generational divide that feels particularly pertinent post-election. This has been a challenging time for kids who disagree with their parents. “Well, you know…” she pauses. “My parents are Republican conservative.”

Bening’s Kansas roots are in sharp contrast to her liberal present. She takes a breath. “I’m very strategic when I talk about politics. Sometimes I choose to, sometimes I don’t. What we do in the work is very important. Within the material itself is where there’s so much power.”

Set against the backdrop of 1970s California, post-punk and second-wave feminism, the themes of 20th Century Women are becoming more salient. “We didn’t make the film thinking that Trump would win,” she says, “but the way that we watch back now is through a new lens. When Obama gave his farewell speech I was crying.”

Role play: with Julianne Moore in The Kids Are All Right. Photograph: Focus/Everett /Rex Features

If you believe Bening, the journey she took to get here is all down to fate. The youngest of a picture perfect, two-son-two-daughter family, she describes her parents as “nice – the perfect word”. Her father was an insurance salesman, her mother sang in church. “They’re still alive,” she smiles (he is now 90 years old and she is 87). “They’ve been married 66 years.” Bening insists their religious beliefs weren’t “sanctimonious”. In fact, they lent her an optimism. “With a positive attitude you can be anything you want,” she says.

When Bening decided to take up acting in high school, her parents were accepting. “I stumbled my way along,” she says. Despite having watched barely any plays growing up, Bening got serious about theatre at the age of 22 and applied to San Francisco University. Her only regret is that she never completed her Masters in Fine Arts. “All I had to do was write a paper on Ibsen’s A Doll’s House. I should have written the damn thing!”

Beginning on Broadway (she was nominated for a Tony), her cinematic break came at the age of 32 with her Oscar- nominated role in 1990’s The Grifters. Appearing nude, she was diminished as a Hollywood “sexpot”, but her acting was always equal parts cerebral and rich in traditional drama. She recalls an article that placed her in an “older women” demographic in Hollywood only a few years later. “I remember thinking: ‘Wow already?’”

Golden glow: with Warren Beatty at last month’s Golden Globes. Photograph: Neilson Barnard/NBCUniversal

Less confident off-screen, she turned down an appearance on The Johnny Carson Show early on. It was the first time she felt unsure of herself. “I thought: ‘If you give me a script, I can do it.’” Even now she’s still surprisingly on edge when playing herself. “I still don’t feel entirely comfortable. But I’m used to it.”

Mills tells me that Bening views self- promotion as anathema. “She doesn’t want to talk about herself, her work or anything – it all makes her queasy. She wants the work to be its own mystery so you have your own relationship with it.”

In person, Bening is intense. Her short hair makes her facial reactions impossible to escape. You feel relieved when she smiles. She’ll laugh and say, “Oh, get out!” when entertained, pushing you away while inviting you closer. It’s that unpredictability that renders her such a scene stealer. Her characters are never simple – they inspire an inquisitiveness among the audience. The credits role not to a conclusive “The end”, but more a leading “The end…?”

Mother dearest: with Lily Collins in Rules Don’t Apply. Photograph: Francois Duhamel/20th Century Fox

Her success, she argues, is driven by great scripts. For American Beauty – her most lauded role – she praises screenwriter Alan Ball’s “knife edge” writing over her own improvisation. “It’s always about the character you’re working on and the point you’re at in your life. A great classical performance happens when the actor is right at the moment when they should be playing it.”

Bening reads a lot of scripts, increasingly for TV, and this year she reunites with husband Beatty for his directed Rules Don’t Apply (out on 10 March). She’s also just finished filming Film Stars Don’t Die in Liverpool, opposite Vanessa Redgrave. “If you can believe that!” she smiles. “That blew my mind, I’m still not over it.”

As she makes her exit, I ask for some motherly advice. “I’m no expert,” she says. “There’s so much pressure on women now to be sexy, to become educated, to have a fulfilling relationship. And – oh yes! – you also have to have children, and you’re supposed to be very happy when you’re doing all of this. It’s impossible. You don’t have to do everything. Just do a little bit at a time.” She closes her eyes tight for a second. “It’s not like we have to be happy all the time. Who wants that? The real thing we all search for is meaning.”

20th Century Women is released on 10 February