John Green talks The Fault in Our Stars at Cadogan Hall, London

What is the collective noun for a gathering of Nerdfighters? What, for that matter is a Nerdfighter? And why are they professing their love for John Green and his book The Fault in Our Stars?

Despite being the catalyst for a worldwide community, seven consecutive weeks at number one on the New York Times bestseller list and, along with his brother Hank, a YouTube superstar, Green was affable - and slightly bashful - as he addressed London’s packed out Cadogan Hall the first Sunday in February.

“Books only work when they are a gift,” he says. “I try to give a gift of the best story that I can make. I need to write the story generously, and that’s the only time when writing works.”

The Fault in Our Stars was ten years in the making for this very reason. The story of Hazel and Augustus, from their first meeting to their last, it’s a heartbreaking and heart-fulfilling love story of simultaneously small and epic proportions. They are smart, intelligent and funny teenagers with wishes waiting to be fulfilled, but for the incurable fault in their stars: cancer.

After a stint as a chaplain in a children’s hospital, Green “started writing all of these stories that were about a children’s hospital, except they weren’t about children or hospital: they were mostly about this 22 year old hospital chaplain, who was like me but super handsome. It was like, which lady doctor will he choose... it was pretty bad. And I think it was bad in retrospect because it was self-indulgent, it was about me, not just literally but also figuratively: I was writing it for me, not for you.”

And so he wrote some other books, and started the Vlog Brothers YouTube channel, and about a million other things too - the Nerdfighting community among them. Nerdfighters are “not people who fight nerds - that would be weird - it’s people who fight for nerds, and celebrate intellectualism and thoughtfulness.” And then he met one particular Nerdfighter: Esther Earl, to whom The Fault in Our Stars is dedicated.

“From the time that I met her she was very sick with cancer, and she died in August 2010, when she was sixteen.” As a result of their friendship, he says, “I came to think very differently about the story that I was writing.” Irritated by stories purportedly about sick people really being about healthy people - “ ‘Oh this sick person came into my life and as a result of having known her, and as a result of her untimely death, I now know to be grateful for every day’ ” - he set out to give voice to sick people themselves.

“In my experience, sick and dying people are every bit as funny and vivacious and interesting and angry and annoying and everything else as other people. I wanted to argue that it is not only long lives that are full and rich and good lives, but that a short life can also be a good life.” And so the handsome chaplain disappeared, and something outstanding took his place. Packed full of metaphor and symbolism and heart and resonance, it is meaningful in a multitude of ways, and I have yet to meet a single reader who has not fallen in love with its characters - or its writer.

After telling us the story of how The Fault in Our Stars came to be, Green reads a short extract and then welcomes his brother Hank to the stage for a quirky, musical interlude. Both brothers are incredibly articulate, answering questions from the audience with such panache and humour that teens and adults alike surely need look no further for better role models. As time begins to dwindle, questions and answers become more frantic, the scene dissolves into mild chaos, the camaraderie between the brothers in evidence. This, though - the mistakes, the goofery - is what makes them human and makes the show both fun and informative, just like Green’s characters, whose faults and imperfections, as much as their inherent goodness, makes us love them and their story.

Overall, he says, “This book is a collaboration between lots and lots and lots of people... You think that books are written by just one person, but that’s not the case at all, no book was ever written by just one person.” This magnanimity, alongside the fact that he treats both his characters and his audience with respect, perhaps explains why, while he essentially writes for teens, he has found just as dedicated an audience with adults. “I think all audiences are vulnerable, all human beings are vulnerable, we’re all afraid, scared and innocent. I don’t feel a particular responsibility because many of my readers are teenagers that I don’t feel for my adult readers. But I do feel a responsibility: I think that I have to write hopeful novels because I believe that the only true novels are hopeful ones.”