In 2009, at a Harry Potter conference, John Green met someone who changed his life forever. Esther Earl was 16, clever and quick-witted. She also had terminal cancer. The writer got to know her family and friends, establishing a strong friendship, but Esther died in 2010. "She helped me understand that people who are sick are not fundamentally different from me or you; they're not separate."

The teenager is largely the inspiration for Hazel, the protagonist in Green's hugely successful The Fault in Our Stars . Sitting on the New York Times bestseller list for seven consecutive weeks, it has become something of a phenomenon. On the day we meet, Green is in Dublin on a 17-city tour of 1,000-seat venues. Tickets sold out weeks ago, and a film version of the book has just been announced. The 36-year-old is relaxed and talkative, explaining that he never wanted to write a "cancer book", and uses Hazel as a mouthpiece to say that "cancer books suck".

“What bothers me is when a book about someone with cancer is not that: it’s really about their friend who is healthy. The person with cancer suffers and dies, but their friend learns important lessons about how to be grateful for everything, and I find that really dehumanising to sick people. They’re not here to impart meaning to other people’s lives.”

Hazel has thyroid cancer, and her parents encourage her to attend a support group. There she meets Gus, a 17-year-old cancer sufferer and amputee. A friendship and tentative relationship begins, with all the awkward newness of first love. Hazel also uses an oxygen tank. It’s a constant, visual representation of her illness. “I wanted to look at the problem of suffering, and if you’re going to do that, you have to be honest about the fact that some lives are short. I love writing about teenagers, because they’re doing so many things for the first time – falling in love, dealing with grief – all against the drama and tedium of illness.”

With subject matter like this, it’s difficult not to veer into Oprah-sanctioned sentimentality. Green’s work is more concerned with authenticity than feelgoodness. In conversation, he regularly refers to the concept of suffering, and he has worked as a minister. Before reading the book, this set off some alarms, but Green avoids offering religion as a salve for illness. He had a stint as a chaplain in a children’s hospital, but he thinks of religion as just another branch of the big questions we all ask. At college, from which he graduated with a degree in English and religious studies, he became fascinated by Islam, philosophy and European history. “A lot of my interest in religion relates to questions of human history that define whether there is meaning in life, so I tried to have both secular and religious world views in the book. I still go to church, but I would not be a good minister,” he says with a laugh.

There is almost something cult-like about Green's following. His fans adore him; he hand-signed the entire first print run of The Fault in Our Stars , and has 1.3 million Twitter followers. Then there's Vlogbrothers, a video blog he runs with his brother Hank and their dedicated fans, called Nerdfighters, who follow their every move. Does he have any issue with the way young people look up to him? "I do get uncomfortable with the role-model stuff, but it's a privilege to get to talk to people I like about stuff I care about. They think you're cooler, less flawed and better than you are. This is also very new to me . . . Before this book I felt like I knew my readers. I thought this book would do awfully, which is why writers should never be trusted to be marketers."

Young-a

dult audience
This is Green's fourth novel, and until now he has found himself writing for a predominantly

young-adult audience. The Fault i n Our Stars has picked up huge numbers of adult readers, and Green is aware that a young-adult tag can narrow the appeal of a book. "Yes, great books can be marginalised, but then you get writers such as Patrick Ness, Mark Haddon and Meg Rosoff, who are finding mainstream adult audiences. I still think of teens as my first audience, but now I get emails from 85-year-old grandmothers who write in all caps, and it's adorable.

"The great thing about teenage readers is that they don't draw a line between high culture and low culture. They're reading Shakespeare in school and watching America's Next Top Model , and they don't see that as discordant."

That same splicing of culture is evident in some of the Vlogbrothers videos. The Dublin instalment included lots of historical and cultural observations as well as toilet jokes. Green engages hugely with online life and social media but acknowledges the problems. “We scroll through the world without really engaging, and it’s a place for distraction. It also furthers our need for affirmation from strangers, and that’s not where you find affirmation: you get it from people who care about you, your actual social network. But we can use the same tools to have better conversations [and] organise protests. I worry that it’s a culture that doesn’t celebrate intellectualism, and the internet has contributed to this idea of fame. It used to be a means, or a byproduct, of doing good work, and now it seems to be the goal. I’ve had a tiny bit of that fame, and it’s horrible. It can feel very isolating.”

Green has been on the road a lot for the past year, such have been the publicity demands arising from the book’s success. Nevertheless, he has started working on “strange, not very commercial stuff” and is keen to return to the day-to-day hum of writing. “Usually, a book comes out and everyone has forgotten about it in a couple of months, and you’re sad . . . This book took over my life for a year in a wonderful way.”

The Fault in Our Stars is published by Penguin