Romance Was Born “Billowing” dress, $590, from David Jones. Credit:Simon Lekias By tackling the potent adolescent issues of social media, bullying, rape, mental illness and suicide in an unflinching manner, 13 Reasons Why is very much a show for, and of, the times. Katherine is still digesting the profound impact it has had on viewers, the overnight fame it has delivered, and the life lessons it has taught her. "I think my story is a bit bizarre," she confesses. "I don't think anyone had any idea [13 Reasons Why] was going to be as big as it has been. I feel like it happened for a reason, and I know that sounds silly. It's definitely been the hardest – but it's been the best – first job I could have ever asked for." When I meet Katherine during her fleeting visit to Sydney earlier this month, she's dressed down in jeans and a black cord jacket. But even in casual clothes she has a prettiness that is reminiscent of a medieval princess, with her trademark brunette curls cascading down her back, breathtakingly beautiful porcelain skin and clear green eyes that meet yours. She dips her head in a sort of curtsey when she shakes hands, is quick to laugh, and given to long, thoughtful pauses as she ponders her answers to questions.

Katherine's own high school experience at Perth Modern, a school for gifted and talented children in innercity Subiaco, was vastly different from Hannah Baker's. The stress of year 12 exams was as bad as it got for her, although she was aware that slut-shaming and bullying – exacerbated by social media – existed. "Playing Hannah brought back the feeling of everyone watching you," she reflects. "I think no matter where you are, high school is always going to be a tough experience, however I think there are things that we can do to make it less painful." While Katherine says she doesn't possess the quick wit or sarcasm of Hannah, she does relate to her character's persistent desire to believe in the goodness of others, even if their actions prove otherwise. She also identifies with Hannah's non-conformist nature. "I was never bothered about fitting in," she says. The daughter of two "extremely intelligent, and very, very hard-working" doctors, Katherine originally had plans to follow in her parents' footsteps. She told a careers adviser in year 10 that her first preference was medicine, followed by politics. Third on her list of career choices was musical theatre. The discussion is the important part about the show. It's about looking at issues and instigating conversation that is needed.

"Even though those first two options were very different, I always wanted to do something that was going to make a difference and/or help people and/or help the world." By the time she graduated from high school, Katherine knew she wanted to be an actor. "I like being able to give voices to characters who are people who don't have a voice, and telling stories that are relevant and reflect on society." But she was rejected from every acting school she applied to, on the grounds she was too young and didn't have enough life experience. So she knuckled down to teach herself everything she could about the craft, juggled three part-time jobs, and found herself an agent. The agent arranged a meeting with some visiting US managers, who set up two video auditions. One turned out to be for 13 Reasons Why. Her casting as the lead is especially remarkable because not only did she win the role based solely on audition tapes she sent over to LA, it is her first professional acting job, and she narrates the series in an American accent. "From what they told me, they auditioned a lot of girls and I think the one thing they wanted to do was capture the essence [of Hannah's character], and they didn't want or need to cast people with a name because it's not a network, it's Netflix."

Kitx “Pussybow” dress, $825. Credit:Simon Lekias 13 Reasons Why has so skilfully tapped into the adolescent zeitgeist that it is reportedly the most popular Netflix series to date, and the most tweeted about show of the year. But the show has also attracted controversy for the way it depicts bullying, rape and suicide. It has been criticised for sensationalising suicide and being overly simplistic about the reasons people kill themselves. American mental health experts have warned it could have a suicide contagion effect, while in Australia, youth mental health service Headspace has reported a "growing number" of calls and emails relating to the show, and cautioned that its content could be upsetting to underage viewers. Some Sydney private schools have banned their students from watching it, while New Zealand introduced a new classification specifically for the show, requiring children under 18 to watch it with a parent or guardian. Katherine says everyone involved in 13 Reasons Why knew it would be controversial, and she welcomes the discussions it has sparked about difficult topics. "The discussion is the important part about the show. It's about looking at issues and instigating conversation that is needed."

Despite the backlash, Katherine takes great pride in the impact 13 Reasons Why has had, particularly in the way its authentic depiction of adolescent issues has resonated with viewers. Among fans waiting when she touched down in Sydney was a girl who thrust a letter into her hands which described the difference the show had made to her own life. "To me, that is the most rewarding part," Katherine explains. "I think we cover a lot of different issues in the show which people relate to differently depending on their own personal history and context. I feel very lucky to be able to have a platform from the show to talk about issues that are important." Being thrust into the public eye so suddenly and so early in her career has meant a steep learning curve for Katherine, who now finds herself walking the red carpet at Hollywood events like the MTV Awards and being approached by strangers in the street. "Suddenly it feels like everybody knows me, and I've just done one thing!" she laughs. Despite being involved in such a massive hit, Katherine displays little of the egocentrism for which the selfie generation is renowned. Unlike many of her peers, she's not particularly invested in the size of her following on social media. "It's not something that I feel I need affirmation for," she says. "For me, it doesn't matter how many followers I have, but if my followers love the show and I'm able to interact with them, that is the biggest pay-off."

The attention is still such a novelty that Katherine is yet to become jaded by it – nor has she let it turn her head, to the surprise of even her closest friends back in Perth. "I'm pretty low key, and I definitely don't consider myself super famous or well known. It's still at the point where it's kind of cool." Amid all the tumult of the past 18 months of her life, the piano has been a refuge of sorts for Katherine. She taught herself to play when she was 16 after she saw Lady Gaga in concert and quickly joined the elite music program at school. When she moved to California to shoot 13 Reasons Why, one of the first things she did was buy a piano for $50. After a tough day filming, she'd come home and play to decompress. "If there's ever a piano in a room, I find it," she says. "I think music is a very personal medium, especially when you write your own songs, which I do. Right now, music is something I'm keeping for myself." She's also keeping other elements of her life private. She reveals she has a 19-year-old sister, but shies away from providing any more detail on her family. And she won't single out any 13 Reasons Why cast members she has become particularly close to during filming, saying only, "All of the cast get along so well, we're all going to be friends for a very, very long time". Asked about boys, she shoots back: "You got anyone you want to suggest?" (She does nominate Winnie the Pooh's Christopher Robin as her first childhood crush, and Orlando Bloom as her second.)

Notwithstanding her new-found celebrity status, Katherine says the greatest impact 13 Reasons Why has had on her own life is what she has learnt from playing Hannah Baker. "I think it emphasised for me the importance of bravery and being bold and unique. It also affirmed that those things are great, they're good things." Armed with that knowledge, Katherine is moving into the next phase of her career. Her first feature film, a screen adaptation of the young-adult novel Simon vs The Homo Sapiens Agenda, which also stars Jennifer Garner, comes out next March. She is now back in the US to shoot the second season of 13 Reasons Why, wiser and more experienced this time around. "There's not really a handbook for being on a Netflix show," she says with a grin. "I know it sounds cheesy, but I just hope to be the best Katherine Langford I can."