Bel Powley knew that the lead role in The Diary Of A Teenage Girl was special. It was more than an extraordinary opportunity for an actress – it also meant something to her personally. Reading the script, she says, "I thought, finally someone is portraying a teenage girl in the correct light.

"When I grew up, all the female characters I used to watch in movies were so 2D and so fake, and I used to be so annoyed. I didn't relate to any of these people. Especially when it came to sex." There were three standard movie options, she says. "It was the virgin waiting for Prince Charming. Or high-school slut. Or the really quippy asexual one." But The Diary Of A Teenage Girl is something different. And Powley, 23, is a revelation in it.

It's being talked of as a breakthrough performance and an Oscar contender, and since the film premiered at Sundance earlier this year, she's an actress in demand.

Written and directed by first-time filmmaker Marielle Heller, the film is based on an illustrated novel by Phoebe Gloeckner, published in 2002. It's the story of 15-year-old Minnie, who lives in San Francisco in the 1970s, and who, at the beginning of the film, has just had sex for the first time with her mother's boyfriend. Minnie – smart, thoughtful, reckless, vulnerable and independent – is the challenging, unpredictable centre of the film, and Powley is in every scene.

Minnie's first words are "Holy shit. I had sex today." She is hungry for experience and intimacy, prepared to take risks, ready to learn from everything. School has little to offer, and her insecure mother (Kristen Wiig) is too busy leading the Bohemian life to pay attention to Minnie or her little sister. Minnie has found a long-distance female role model, comic-book artist Aline Kominsky, whose work inspires her to express herself with pen and paper; the movie makes deft, judicious use of animation to convey moments from Minnie's fantasy life.

Minnie is drawn to Monroe (Alexander Skarsgard), her mother's 35-year-old boyfriend, and they embark on a covert sexual relationship. This problematic liaison is not the defining aspect of the film: The Diary Of A Teenage Girl is about Minnie's fearless search for who she is, physically and emotionally. When she examines herself in a mirror, she seems both unselfconscious and self-aware. The movie explores her in all her complexity; her childish side, her impetuosity, her willingness to act on her desires, her growing sense of maturity.

Powley – who played Princess Margaret in the World War II comedy A Royal Night Out – knew that she loved this vision of the heightened, extreme experience of adolescence, but all she wanted in the first instance was to be considered for the role. "I got sent the script by my American agent, and I had been with them for four years and not booked anything. I felt incredibly passionate about it, but I didn't know how I was going to be in this film. I was sending tapes from England to America, thinking 'is anyone even watching them?' ''

She recorded a scene, "and at the end I added this bit on, speaking to the camera, saying, 'Hi Marie, I'm Bel Powley, you don't know me, I'm in London, I can't be in Hollywood, but I really need to speak to you about this movie because there are so many things in it that I relate to, and I just want to have a conversation with you'."

Powley has talked of "going through a rebellious stage" in her mid-teens, but it was nothing like Minnie's. What appeals to her about the film's treatment of adolescent rebellion is that it's not punitive, and it doesn't impose life lessons: Minnie is a character in flux, figuring things out for herself, and there's something inspiring about this.

Powley, 23, hadn't really planned on acting as a career, although her father is an actor and her mother a casting director. She did drama as a weekend activity, and ended up being cast as a youthful secret agent in a BBC kids TV series called MI High. She had thought she would go on to university. It was theatre that changed things for her. She made her West End debut in Polly Stenham's play Tusk Tusk, about three children left to their own devices in a London flat, and was nominated for an Evening Standard Award for Best Newcomer in 2009. She went to Broadway, appearing in the 2011 revival of Tom Stoppard's Arcadia, in which she played a brilliant young aristocrat with a gift for mathematics.

As it turned out, Powley's fierce sense of identification with the role was familiar to actress-turned-director Heller, who had been working on The Diary Of A Teenage Girl for eight years. It had been, Heller says, a long-term love affair. She read Gloeckner's novel and "immediately fell in love with the character.

"I felt she was the most complex and honest version of a teenage girl I had ever seen portrayed in a book. She was a hero to me. So honest and curious and earnest."

Heller, 35, started by turning the book into a theatre piece in which she played Minnie. She then started working on a film script. Being close to the project for so long, she says, gave her certain advantages. It was also easier than she thought to see the character played by someone else.

"I was so ready to make this movie, and felt so clear that I needed to focus on directing; I wanted to find the perfect person to play this part. And when I found Bel, she understood her so deeply in her bones in a way that was so great, I knew it was a sharing and a collaboration that we could grow from. I was very conscientious about never bringing up the fact that I had played the part. I never reminded her of it. We had a classic director-actor relationship."

On set, Heller says, she and Powley were "really chatty and constantly divulging all these secrets about our past, and the ways we related to these characters".

Beyond the complexities and contradictions of Minnie, Powley also brought, Heller says, "a physicality that I had not totally experienced before".

"She came in with this all-limbs, gangly way of playing Minnie, and it was so perfect. She looked like a little baby deer learning to walk. And we put her in pants that were a little too tight for her, shoes she didn't quite know how to walk in, and she was embodying that time of your life when your body has shifted and you're not quite comfortable in it yet."

There was a solid period of rehearsal before shooting started, Powley says. "We just went through the script, and worked out emotionally in each scene where Minnie and Monroe were, so we understood it. That really freed us up to find new things when we got on set, because we had a basis for where we were."

The first scene they shot was the one in which the sexual relationship between Minnie and Monroe is set in train. "It was crazy that it was the first thing we shot, in some ways," Heller says, "because it's such a powerful and emotional scene that we had to get all of the nuances right. In some ways the entirety of their relationship was embodied in that scene. They're both walking up to the line, and not sure who's making the first move, they're kind of doing this dance with each other, where one is pushing the other and then it finally explodes with tension. If you can't go along with that scene, you can't go along with the rest of the film."

The Diary Of A Teenage Girl is a period piece, and the fact that it's set in the 1970s in San Francisco is important to the way the story unfolds, Heller says. People searching for an alternative life flocked to the Bay Area, and the influence of '60s counter-culture was still strong.

For Powley, it was important to absorb the setting in a very particular way: she wasn't hooked on detail, but focused more on how things felt. "The only thing I really did to prepare and learn about the '70s was to download every single one of Janis Joplin's tracks and listen to them for two months," she says. "Because you know from the book that she's Minnie's favourite artist. I didn't want to watch too many '70s movies ... I didn't want another actor's interpretation of the '70s to affect mine. It was more when I got to San Francisco, two weeks before shooting. It was taking in the city, walking around, walking where Minnie would have walked, getting into the bellbottoms, cutting my hair into bangs made me feel like I was in the period.

"I started in theatre, and my way of working in theatre is creating a character, spending four weeks discussing it, four weeks doing background work, improving scenes that aren't in the text, working out what happened to them when they were a little kid, then going on stage and letting all those things that I've decided for the role be affected by the context and the other actors in the play. I've applied that to film.

"I spent two months creating a character down to the way she talks, she walks, she holds herself. I was channelling Minnie for every moment of the movie, even when I was sitting there not talking, or standing in front of the mirror examining myself, I was always thinking from her point of view."

Her theatre background remains crucial, she says: "oh my God, 100 per cent. I need to always go back and do a play every year or every two years to ground myself. I'm trying hard to find my next play." In the meantime, she has completed another couple of movies, and has several in the pipeline. One is A Storm In the Stars, a drama about Mary Shelley (Elle Fanning) and her sister (Powley) and the writing of Shelley's Frankenstein. It's being directed by Haifaa al Mansour (Wadjda) the pioneering Saudi Arabian filmmaker who studied in Sydney before making her first feature. "I'm really excited about that, another film with an amazing female director. And I'm going to do a movie called Ashes In The Snow, set in Lithuania during the '40s, about a girl who gets sent to a gulag."

Beyond that, Powley plans to keep The Diary Of A Teenage Girl in perspective. "I see it in terms of the way you see any career, you move up another rung on the ladder, you work hard and move up to the next one. I'm going to keep my feet on the ground and keep making good films for women."

At the same time, she says, laughing, "it was such an amazing leading role, and leading roles like this don't come along in many actors' careers at any age, let alone this young, I feel like it will never get any better than this. It's all downhill from here."

The Diary Of A Teenage Girl opens on September 24.