Bel Powley in "Diary of a Teenage Girl"

Though Marielle Heller had passed the formative years that she recounts so tenderly in her feature debut “The Diary of a Teenage Girl,” making a film meant an entirely different coming of age. An actress when her younger sister gifted her a copy of Phoebe Gloeckner’s graphic novel, she became a playwright when she became so enamored of Gloeckner’s account of a 15-year-old’s blossoming as an artist and a sexual being that every square inch of her place was covered by a photocopied page from the book, annotated and reshuffled so it could be staged off-Broadway while preserving the hazy, reminiscent quality that made her love it so much in the first place. Still in her twenties when the play hit the boards at the 3LD Art & Technology Center in lower Manhattan in 2010, Heller wasn’t too old to play its lead Minnie, burrowing even deeper into the world Gloeckner created.

“I would’ve expected at some point I would’ve been bored with it, but that part’s never happened, so that always felt like a good sign,” Heller says now, with a slight laugh.

It was, since it’s hard to imagine anyone else give as much care and attention to make the film adaptation of “Diary of a Teenage Girl” to work as well as it does. Aesthetically a mix of earthy animation and beautifully burnished live-action and tonally exuberant and contemplative, it’s perfectly suited to convey the mix of occasionally incongruous impulses and actions of its high school heroine, thrust into adulthood by an affair she has with her mother’s boyfriend. Further confused by an era just between the free lovin’ ’60s and the cultural conservatism of the 1980s with the echoes of the Patty Hearst kidnapping saga never far away in her hometown of San Francisco, Minnie (played by an iridescent Bel Powley) is left to her own devices to figure out who she is with no easily identifiable role model, given her mother (Kristen Wiig) is preoccupied with her own complicated relationships with men and her lover (Alexander Skarsgard) should know better, but doesn’t. Instead, she turns to a tape recorder to keep track of her thoughts, a sketchpad to express herself and her favorite underground comics characters to talk to.

There’s no doubt that Heller, who shares a background with Minnie as a self-described “child of hippies” from the Bay Area, brings a historical authenticity to the film, but it’s the emotional authenticity and compassion that extends to all of the characters in “Diary of a Teenage Girl” that makes it so impressive and trenchant. Shortly before the film hits theaters, Heller spoke about its eight-year journey to the screen, having the confidence to become a writer/director without any notable prior experience and potentially giving audiences the same inspiration that Gloeckner’s novel gave her.

Bel Powley in "Diary of a Teenage Girl"If this has been eight years out of your life, what’s it been like to grow up with this project? Has your perspective on the story changed as you’ve gotten older?

I don’t know that it’s shifted or changed all that much. I’ve had to grow and learn a lot about how to actually make a movie through this process, but my love of Minnie has never wavered and my feelings about these characters and why this story is important has always been there. The things that resonated with me eight years ago still resonate with me today. If anything, I just had to become clearer and clearer about the story I wanted to tell.

Since you performed this every night on stage, did that give you some special insight in how to ultimately bring this to the screen?

Definitely. Playing a part, you get to know the inner drive of a character from inside out. It’s in your bones and obviously, my Minnie was different than Bel’s Minnie in some ways, but when I met Bel, I felt she understood her in her bones too, so we could talk about all of the inner workings of this character in the same language because we both innately understood who she was inside.

Kristen Wiig and Bel Powley in "Diary of a Teenage Girl"Did Bel bring something to it that you weren’t really expecting?

I think so. She came in with this physicality of the character of being all limbs – of someone who’s had a growth spurt and hasn’t learned to live within their body quite yet. That affected her walk…also ‘70s platform shoes and tight pants will do that too. [laughs] But that was such a wonderful, weird physicality that we hadn’t really necessarily talked through, and it was so perfect for the part because when your body’s changing and shifting, you don’t know how to quite move yet in it. There’s an awkwardness to it that she just full-on embraced and I loved that.

One of the most striking things to me in the film is when Minnie takes a polaroid of herself and says, “I don’t see myself here.” Not to belabor the metaphor, but how conscious of you were of that idea of Minnie looks through the lens of various media and she’s not seeing herself?

Very conscious. I see her as a character who’s always trying to cope with and deal with her life through art, whether it’s taking pictures or through drawing, recording or whatever it is. She’s trying to get to know herself through this process and one of the ways she tries to do that is by seeing how other people see her and how does she see herself – is she beautiful? Does she look different now that she’s had sex? Can her cat tell the difference? Can she tell the difference? What does her body look like? How do other people see her body? So that photograph became this physicalization of exactly that journey of “What did I look like in this moment, right after I lost my virginity?” And “Can I go back there and am I different now?”

Part of that is exploring this idea that women are given this bullshit lesson that virginity is this thing you have to guard and once it’s gone, you’ll never be the same. There’s this feeling people will know something and also this feeling, is it having sex that makes you an adult? A woman? So all of those things are wrapped up in that idea that she’s looking at this picture, trying to get to understand herself in some way.

Bel Powley, Kristen Wiig and Madeleine Waters in "Diary of a Teenage Girl"Just a day before this interview, it was announced that the film would be given the equivalent of an NC-17 rating in England, and when I saw it at the LA Film Festival, it was interesting to hear Bel note how she’s often asked about the nudity, but always indirectly. Have the conversations you’ve heard this film inspire about teen or female sexuality reinforce what you may have believed before making it?

It’s interesting because I don’t feel the nudity in this film is gratuitous at all. The most nude scenes are not the ones that are sexual. They’re in non-sexual situations when she and Monroe are in the fight or she’s just looking at herself in the mirror. So I find it so ridiculous that there’s even controversy about it because there shouldn’t be. I know it’s so taboo to talk about female sexuality, and it’s especially taboo to talk about teenage girls wanting to have sex. Society would like to pretend that’s not true, but the truth of the matter is most teenage girls are having sex and all of them are thinking about it. Pretending that’s not true is just sheltering them for sheltering’s sake, which I think is just crazy.

But we’ve had great conversations coming out of this. I think it makes some people uncomfortable, but it’s a really great conversation to have because movies have been made about young boys having sex forever. You can point to movies from “The Graduate” to “American Pie” and boys have been able to feel like they are not alone and they are not freaks for a long, long time, no matter what they do sexually, so I think girls deserve the same.

Alexander Skarsgard and Bel Powley in "Diary of a Teenage Girl"After carrying this with you for so long, what was the first day of shooting for you like?

The first day of shooting was incredible. On the first day, we shot the scene where Bel and Alex – Minnie and Monroe – go to the bar and she bites his finger. That was our very first scene we filmed. Then we filmed the scene of them at the beach where they say goodbye to each other, so it’s like the bookends of their whole relationship. It was this huge, massive day of really emotionally intense scenes we had to nail. We couldn’t have any first day jitters and we had to just hit the ground running. The chemistry between Alex and Bel was there and that was what we needed to get right. We needed to get the tone between them right and from that first scene, we just got it all – every nuance, twist and turn was in that scene in the way they played it.

I remember [cinematographer] Brandon [Trost] turned to me at the end of the day and he said, “I think that was the best first day of a movie I’ve ever had.” It felt amazing. We just felt like we nailed the tone and we could see the whole movie. I didn’t want to jinx myself, but I felt really confident coming out of that first day.‎ It was one of the best days of my life.

It’s such a different film for Brandon…

Yeah, I love telling people, “Yeah, the same person shot this shot “The Interview,” and “Neighbors” and “MacGruber,” my husband [Jorma Taccone]’s movie, but the truth of the matter is Brandon is an artist. I think he’s found his niche in the giant comedy world, but he loved working on this material. He wanted to do something different, so he loved making every single shot as beautiful and artistic, but also correct for the emotional moment as it could be. He just nailed it.

Marielle Heller on the set of "Diary of a Teenage Girl"Were the walking and talking scenes with  animation difficult to manuever?

Yeah, that was one of the hardest scenes – of her walking with the animated character down the street because we were blocking all this traffic and it was one of our longest shots. I was yelling the other lines to Bel [while] trying to run behind the camera. There were crazy people running in and out of our set. That was like juggling knives. That was a cuckoo day.

Does the film mean something different to you now than when you first started down this path?

Coming out on the other end of a film, it’s bittersweet. The thought of this movie not being in my life and this story being over for me is in some ways really sad, but I’m just so proud of it and what we accomplished. The fact that it’s going to be out there in the world for people to see just makes me so happy.

“The Diary of a Teenage Girl” opens on August 7th in Los Angeles and New York.