Books can be life-changing and never has there been such a case in point as when Marielle Heller was gifted a copy of The Diary of a Teenage Girl eight years ago. Besotted by the story and how it stood as a sui generis to the anxieties and tribulations of female adolescence, the UCLA and LAMDA theatre graduate set about acquiring the novels play rights before mounting it off-Broadway in 2010, where she also starred in the lead role.

After a critically acclaimed six-week run, author Phoebe Gloeckner was sufficiently impressed to grant Heller the full movie rights which would also mark her feature directing debut. Starring Brit newcomer Bel Powley in the lead role along with Alexander Skarsgard and Kristen Wiig, The Diary of a Teenage Girl premiered at this year's Sundance Festival where it stood as one of the break-out films, drawing acclaim for its unique female interpretation of the coming of age genre, while scooping tangible recognition in the form of the prize for best cinematography. Since then, it went on to take the title of Best Feature Film and Best International Feature Film at the Berlin and Edinburgh film festivals respectively.

Speaking from LA, director Heller caught up with the 405 about what the story means to her, the '70s, and some of the current issues surrounding Hollywood.

Photo courtesy of Sony Pictures Classics.

When did you first read the graphic novel?

My sister gave me it as a present eight years ago. She didn't do so with the intention of me actually doing anything with it; she had read it herself and loved it and thought I would too. I was blown away and it just felt like the most honest depiction of a teenage girl and coming of age sexuality story that I'd ever come across. I was so compelled that I didn't even realise it was something I hadn't seen before. After that, I started stalking Phoebe Gloeckner, the author, and her agent, until I finally acquired the theatrical rights.

You had already spent 3 years developing The Diary of a Teenage Girl as a Broadway play. Was it always your intention to one day translate it into a film?

No! I'm a theatre person first and foremost so the idea was always to make it into a play. It was such a positive experience and I felt so happy with how it had gone. When I came out on the other side I had the feeling that I still wasn't finished with Minnie or the story and that's when I started conceiving it as a film. It really wasn't something I'd intended at the beginning.

Photo courtesy of Sony Pictures Classics.

How much did you rely on the script from the play to inform the screenplay?

I had to change a fair amount. It was a five-character play with a lot of magical realism and was kinda its own thing. The process of adapting the book into a play and then the play to the film was really challenging because I loved the book and I had so much reverence for it. At one point I had to abort being so true to the book and Phoebe and try let it become something new. The form of theatre is really different to the form of a book and the form of a film is really different to the form of theatre.

When reading the synopsis I think people might naturally assume that Monroe (Skarsgard) would assume the role as this dominant figure while Minnie (Powley) would be recessive and impressionable, yet at points it's the complete opposite. The film I think not only deals with the pressures of being a teenage girl and growing into womanhood, but the wider complications of assumed social dynamics.

I think that's exactly it. We're much more comfortable in our society with narratives that are black and white which paint a very clear victim and a very clear predator where we can have a moralistic judgement of who's right and who's wrong, but life isn't like that and the truth of the matter is that most situations are complicated and people aren't all good or all bad; they're human and interesting and have complex ways. I think exploring all of that makes for a more interesting story as well. When it comes down to it, Minnie is being taken advantage of but she doesn't feel like she's being taken advantage of, so we shouldn't either. The film is from her perspective so if she doesn't feel like a victim then we shouldn't feel like she's a victim.

One of the endearing things about the character of Minnie is how she becomes so hopelessly preoccupied with Monroe and sex to the point it begins to affect her relationships with others while her grades in school begin to suffer. Even as a guy in my 20s, I felt I could relate with her and it reminded me of the anxieties of being that age where everything else seems to pale in significance and makes you feel sort of small.

Totally, I think we can all relate to that and I like that you as a guy can do so with this part of her character because I've been relating to male protagonists my whole life. I think Minnie is a really relatable protagonist because everybody goes through that major hormone jump where suddenly all you can think about is sex and romance, and it's as if everything else falls away even if you know you should be focussing on other things - like Minnie with her art.

Photo courtesy of Sony Pictures Classics.

I think it's a real shame that it's been given a certificate 18 here and could potentially bypass the younger audiences it would resonate with the most, particularly when movies loosely similar such as Catfish have been certified 15.

I was really surprised by that because it's stricter than what we got in the US and I didn't think there was any chance of that happening, but I hope that young men and women do sneak into the movie and go see it anyway. I think it's a shame that an all-male board get to decide what young women are allowed to consume. We can pretend that it isn't true but in reality most teenage girls are having sex and not talking about it won't change that. Banning people from going to see this movie is not going to make the fact that girls are having sex and thinking about sex any less true. Good luck trying to shelter them, but it's not going to work.

Yeah, I think when you look at the scenes which were flagged up in the overall context of the film then they make perfect sense. There's nothing there which comes across as wanton and if anything it makes the rating system seem out of touch...

I don't know why but I think people are still afraid of teenage girls and their sexuality, with the rating handed out due to fear more than anything else. This is a film made by women about true women's stories and it's not gratuitous. It's really sad that we're much more comfortable with things like rape scenarios in films. If this had been a movie where she was very clearly being raped and not enjoying sex at all then it would probably have gotten a lower rating, but because it's about a girl who's exploring and enjoying sex it gets a worse rating. I think that's very telling and unfortunate.

Photo courtesy of Sony Pictures Classics.

Why do you think it's taken so long for a movie from a female perspective like this to come to light? I read Alexander Skarsgard saying the other day that sexism is still a big problem in Hollywood...

I can't say why because I don't think there's any big conspiracy theory. Hollywood is run by men in general and so they're going to tell the stories that they can relate to. Until we have more female filmmakers and storytellers being given a chance then we're not going to have more movies like this. I don't think it's malicious, I think it's the reality that a man wouldn't make this movie because it wouldn't be his life or experiences and he wouldn't know how to do it.

Even though the themes of the film aren't exclusive to a particular time, it's hard to imagine it working so well if it was set in an era other than the '70s. The free-spirited, experimental nature of the baby boomers came through really well.

For me it was important to keep it set in then, not just because of the book, but because it was a really interesting cultural time to explore, particularly in the San Francisco area. These stories still exist today, of course, but having it set then hopefully let audiences come in and let the story play out with a little less judgement.

I noticed the Patty Hearst trials feature as a running background news item throughout the film. Why did you decide to feature that particular case?

When I was doing research for the film - and there's a brief mention of it as well in the book - I realised that the Patty Hearst trials would have going on at the exact same time. Everybody would have been watching it as it would have been the cultural touchstone in the same way that the OJ Simpson trial was back when I was at high school. I read this one article which said something that I felt was really parallel to the story that I was trying to tell in that the trial was so focussed on the idea of personal responsibility versus societies responsibility and how our actions are our own and that it's up to us to take responsibility.

This article was posturing that if this trial had taken place 10 years earlier in 1966, public opinion would have been that Patty was the victim and that society failed her by not taking care of her, whereas if it took place in 1986, public opinion would probably have been more along the lines of "she's an individual, she should have pulled herself up by her bootstraps and it's totally her fault." In '76, we were really somewhere in the middle of those two eras and were having that exact cultural discussion; what is personal responsibility versus societies responsibility? How do we care for our young people and how do we care for our women? I just found it a really interesting question and a neat parallel with Minnie's story. Is she entitled to her own sexuality or is it Monroe's or her mother's responsibility?

Photo courtesy of Sony Pictures Classics.

UK audiences might know Bel Powley from some TV work, but she's a relative unknown really. How did you discover her?

She submitted a tape from the UK which was just incredible and at the end she included a little extra part where she looked into the camera and spoke to me about why the movie meant so much to her and why she wanted to be involved which I found really touching. I care so much about the character and it was important that the people I brought in were equally as passionate. As well as that, she also had to contain all of these conflicting attributes such as being good at comedy and drama, while looking like a girl but also a woman. She needed to be awkward and sexy; she needed to be geeky enough to grow up to be a comic book artist yet striking and beautiful. It felt like I had this impossible list of demands of what I wanted the character to be, but then Bel came along and embodied all of them.

After playing the role of Minnie yourself in the play for so long, was it hard to segue control and not be too didactic to Bel?

I thought it was going to be more difficult than it actually was. It became really clear to me that Bel's Minnie was different from mine, but she understood her in the same the same way that I did and we both felt this responsibility to this fictionalised version of Minnie that existed outside of us, so we wanted to honour and do right by Minnie. I could see that she had a lot of respect about who this girl was and that's what made it really easy to let go. I remember a moment when our costume designer looked at me and said "can you remember our life before Bel?" and I really couldn't. She was so perfect for the role and I can't imagine anyone else playing her.

This story and its characters has been such a big part of your life for so long now. Do you have any idea what your next move might be after the films release?

Luckily for me the film is ending just as I've begun a new chapter in my life as a mother. I have a little 7 month old baby so that's helpful because otherwise I think I'd be losing my mind. I'm also working on a lot of really exciting projects. I just directed an episode of the TV show, Transparent, and I've also signed on to a film about Ruth Bader Ginsburg starring Natalie Portman.

The Diary of a Teenage Girl goes on general release in the UK and US on Friday.