An hour before I leave to meet Cara, there is a change of plan. A car will pick me up and drive me to her photoshoot, but instead of conducting the interview on set, Cara and I will hop into another car and talk on the way to her next appointment. Then I will be dropped at some as-yet-unknown location, at which point a third car will appear and take me home. It's all very Hollywood. I've lived in Los Angeles for 20 years, and this is the kind of thing that can happen when working with celebrities.

By the time the first driver and I arrive in dusty, mountainous Shadow Hills, I'm equating it to the Australian Outback. We sail past a nondescript front gate, then circle back and begin our ascent up narrow roads that snake up to where the views and the house – a white Moorish castle – are breathtaking; where the sky seems bluer against all that white.

I'm told that Cara is inside Furst Castle – as it's known – still shooting the ELLE cover-story photos, and so I sit outside in the sun and wait for her. I'm a bestselling author of books for young adults, including Holding Up The Universe and All The Bright Places, and the entire time I wait for Cara, who has just written her first book (she describes it as 'a twisty coming-of-age story about 16-year-old friends Red, Leo, Naima and Rose, who are all trying to figure out who they are and navigate the minefield of school and relationships'), I'm thinking about how many of my readers would love to be here right now.

Cara Delevingne photographed for ELLE September 2017 by Kai Z Feng, styled by Charlotte Stockdalepinterest
Kai Z Feng

At 25, Cara is a role model. She is an idol and an advocate. The best kind. She is known for her honesty. She famously hates being labelled, whether in her career or for her sexuality. She speaks her mind about the issues affecting young people today – mental health, suicide, self-harm, body image, sexual identity, bullying – and she lets them know they aren't alone. It's the same message I've been sharing with my readers around the world, so it's one I'm invested in. I know how important that message is to teens. I know how much they need and deserve to hear it.

I sit in the sun and hope Cara is as genuine as she appears. As someone who spends her life giving a voice to teens who are struggling, authenticity is important to me. Thirty minutes later, I'm invited inside to meet her.

'Dude,' she says. 'I'm so sorry about the change in plans.' She hugs me hard, and my first thought is that she is small. Delicate, fine-boned, lovely. Those eyes are even more striking in person. She looks like a warrior, like Joan of Arc, her hair shorn short, but there's a fragility about her — maybe the smallness – that makes me want to protect her. There's something else about her, though. Something that you sense within about a minute of being around her. An underlying strength and ease. She's at home with herself and her surroundings, and has a real-ness that instantly makes me feel at home as well.

Those eyes are even more striking in person. She looks like a warrior, like Joan of Arc

Her driver has brought her food from In-N-Out Burger, the cult California fast-food chain, which she is ecstatic about. We follow him to the car and climb into the back seat. Cara's had a long day and still has hours to go. She yawns and apologises for her brain, which is jetlagged and in need of food. I tell her to eat. She opens the bag, breathes it in, closes it back up and says she'll wait. She wants to focus on the interview, and the food will only distract her.

For a mile or two, we chat about the glory that is In-N-Out Burgers, the history of Furst Castle, and the heat of Los Angeles versus the chill of her native England. It's at this point I realise, cruising through one street after another, that I'm not paying attention to the questions on my list. We're just talking and laughing, which is easy to do with Cara. You forget you're hanging out with a celebrity. You feel as if you're hanging out with a very smart and funny friend. A friend who sees and feels the world intensely. But, because our time is limited to this car ride, I break out my questions.

First things first, her debut novel, Mirror, Mirror, a young adult mystery-romance that she's co-written with bestselling British author Rowan Coleman. I tell her I've read it, and that it's good. 'What? So you've read it?' She is genuinely surprised. Her eyes go wide. 'Please tell me, because I haven't spoken to basically anyone else who has read it.' She seems excited, eager, and hugely flattered. I tell her what I love about it – the twisting plot, the relatable characters, especially the main one, Red. You can feel her excitement. Eyes sparkling, hands waving, she talks fast and with obvious passion for Red, who first led her to the story.

'Red was the initial character we came up with, then everyone else came afterwards. It was the core group of friends you had when you were younger that we wanted to explore, and the essence of growing up in London. The idea was first based on – what's it called – Alice…'

'Go Ask Alice?'

'Beatrice Sparks' Go Ask Alice! Thank you. Not as dark as that, and obviously modern-day, but the same insecurities of not knowing who you are. The trials and tribulations that come with being a teenager. The kind of book that sticks with you.' I tell her I feel Mirror, Mirror is one of those. 'That's everything I've ever dreamt for this book.'

You feel as if you're hanging out with a very smart and funny friend. A friend who sees and feels the world intensely

I don't tell her that, before I read Mirror, Mirror, I was sceptical. So many celebrity books are written by other people, and so many celebrities claim to feel passionate about a cause but, in reality, aren't. How did this international model, actor and musician come to write a novel in the first place?

'I've always had this wonderful connection with teenagers, from when I started with social media. Just having girls message me being like, "I'm really dealing with the pressure of my thoughts, my friends, eating disorders." That kind of thing, where I was like, I have an opportunity to really be there for them and help. You know, be a voice for teens and be honest as to how I suffered as a teenager.'

There is something in her voice I recognise: passion and empathy. I can see it in her face – she feels like I do. She realises what a responsibility she has. You can't fake that. I want to know what she was like as a teen, and if there were any books or music that made her feel less alone when she was growing up.

'I listened to Fiona Apple a hell of a lot. She has such an incredible way of articulating how her mind and her emotions work. I think the problem for me was that I didn't discover the incredible power of books until I was older. Just because books, to me, felt like school. And I had such a fear of school and exams because I wasn't particularly good at them – my brain didn't work in that way. It took me a while to be, like, "Wow, books are the most incredible thing."'

Was there a book that she remembers discovering as she got older? The first book that made her go: 'Oh! You know what? It's not just about exams and school'? She stares out of the window, clearly thinking this over. After several seconds, she turns back to me, her face lit up. 'The book I've probably read the most is Lena Dunham's [Not That Kind Of Girl]. Like, a million times, because I love that book; her honesty and raw humour. I really like darkness, because it's crazy to see the pain and the things that people go through without telling anyone.'

Cara Delevingne photographed for ELLE September 2017 by Kai Z Feng, styled by Charlotte Stockdalepinterest
Kai Z Feng
Cara Delevingne photographed for ELLE September 2017 by Kai Z Feng, styled by Charlotte Stockdale

When I ask whether she did any creative writing when she was at school, she says: 'I mean, we all had to do creative writing and English, but I didn't enjoy it as much because I felt forced. Whereas now, that's all I do. I've just spent a week by myself in Germany, walking around in the Alps, writing and sitting on top of a hill. That's when I feel like I get the most out of my days.'

We agree: escaping into nature gives you the balance you need for creating.

I ask if her acting has influenced her desire to create her own stories. It's clear that she's a storyteller – through music, acting, even modelling, and now via the written word. She nods, fixing me that direct, no-bullshit gaze. 'For sure. If I hadn't gone into acting, I wanted to be a child psychologist or a therapist. The way people are with each other in relationships – I find that so interesting. When I started acting and putting myself in other people's shoes, it made me much more aware. Storytelling brings people together. Every movie I've done has made me realise certain things about myself and the way that I can connect with other characters.'

So what was the book-writing process like? Once she decided to pursue writing a novel, she met with several different possible co-writers. When she met Rowan, she knew. It was, she says, fireworks. 'It was like a chemical reaction. She said to me, "I didn't know how good you were going to be at this, but I'm really impressed." Coming from an incredible writer, that was amazing to hear.'

If I hadn't gone into acting, I wanted to be a child psychologist or a therapist. The way people are with each other in relationships – I find that so interesting

And so, Mirror, Mirror. Is there a character she identifies with most? 'Red; she's a drummer, a tomboy. To be honest, I feel like I've put little bits of me in all of them.' She goes on: 'When I was a teenager, I was in a band. It's so important for teens to have something like that outside of school, where you can connect with friends, express yourself or meet kids who aren't in your social circle or clique, or "the cool kids" or whatever. Because those are the friends that you end up having for life.'

'How important is friendship in your life?' I ask. 'So important,' she says. 'One of the reasons I wrote this book was because I wouldn't be here today without my friends and family. My closest friends, the people I call my dearest friends, are like my family. They've helped bring me up.'

I stop being aware of the fact we only have a few miles left to finish the interview. I'm losing sense of time because I'm enjoying talking to Cara. This fascinating complex, real person.

'What do you think is the most difficult aspect of growing up, going from girl to teen to woman?' I ask. 'Which do you think is the worst stage, if there is one?' She replies: 'When you're in it, every moment feels like it's the hardest. When you look back, it seems like it was easier. It's difficult to appreciate, because there's so much going on in terms of hormones and pressure [as a teen]. So much pressure! And that's what I wanted to capture in the book. With social media, and the pressure of having to be "perfect" – you're just trying to find your identity. The pressure of that is huge.'

I wonder what has been the trickiest age for her to navigate. She exhales, as if she's been holding her breath for a long time. Shakes her head. 'I mean, I am so lucky, man. I'm living my dream right now. So far I've had an amazing, incredible life, experiencing so much. I don't know. Probably a teenager, especially with school and not giving yourself a break. And not asking for help, or saying, "I'm really struggling." That was the biggest lesson I had to learn: to communicate my emotions properly. I'm still learning how to do that.'

Was there no one she could talk to and turn to? 'It felt like I was completely alone and I couldn't express myself because I felt ashamed of my emotions. I want to make sure kids realise that emotions and vulnerability are important and should be spoken about. We're alone, but we're all in this together. We're all humans, going through the same things.'

This is a woman who has spent a lot of time in the public eye. I want to know what it is about that experience that has helped her understand what these kids are going through. She takes her time answering this question, her brow furrowed. 'When you're a teenager, you're looking for someone to idolise. Seeing the effect people like me have on teenagers now has really made me conscious of kids having strong, positive role models who are trying to do good outside of themselves.'

I think about the teens I hear from daily on social media, the ones struggling in silence, and I suspect that Cara sometimes is struggling, too. Then I ask her what makes her feel empowered. It really can be anything – big, small, insignificant, profound. 'Being alone or going for a walk by myself, writing or just doing things for other people. Helping people. You can find things that empower you in everything. It's just about being in the moment and being yourself. You can't think about being empowered – it's a feeling that comes from inside.'

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Cara says she's pretty comfortable in her own skin. 'Accepting that is an everyday process,' she says. 'It's about loving yourself, making sure that you've taken time to respect yourself. It's about not blaming yourself for everything.'

When you're a teenager, you're looking for someone to idolise. Seeing the effect people like me have on teenagers now has really made me conscious of kids having strong, positive role models who are trying to do good outside of themselves.

What is the message she wants readers to take from Mirror, Mirror? 'That life is a beautiful mixture of wonderful disasters, but to truly love yourself is so important. Sometimes, you won't be able to go to someone, so you need that strength in yourself to know that you can carry on and be all right. To know that you can reach out and talk to other people. There are misunderstandings and things go wrong, but don't judge people. Try to understand each other, and try to see where other people are coming from.'

And it's at that moment, perfectly timed, that we roll into the car park of Laurel Canyon Country Store. This is where Cara and I will say goodbye. The ride has gone far too quickly, but I have only one question remaining. I take off my seatbelt and turn to her. 'Has this experience inspired you to write more?'

The intense expression fades away. Her face is suddenly as bright at the Californian sky. 'Yes. I have so many more things I want to write.


Cara Delevingne is the cover of ELLE's September Issuepinterest
Cover shot by Kai Z Feng, styled by Charlotte Stockdale

This article originally appeared in the September issue of ELLE UK