PITTSBURGH — Most movie sets orbit around one key individual. Usually, it's the director or the star. Occasionally, it's a heavyweight producer. Seldom is it the writer of the movie's source material. But John Green is not just any writer.

On a crisp day in early October as cameras rolled on Fox 2000's film adaptation of his 2012 novel "The Fault in Our Stars," a bestselling love story about two wry, cancer-stricken teenagers, the 36-year-old author was exerting a strong gravitational pull.

One minute, he was sitting with Fox honchos in director's chairs, sharing his ideas for marketing the movie ("I can be in as many places as you can fly me in a single night"), the next, he was regaling the executives with the wild story of the preposterous ending he first wrote for the book (It involved a road trip to Mexico and a battle with narco-terrorists.) 

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After an emotional scene featuring young actress Lily Kenna, who shaved her head to play protagonist Hazel Grace at age 11, he pulled her into a hug and thanked her for her performance. "I think you did an awesome job," he said.

With an infectious enthusiasm for those around him, a facility with impromptu storytelling and a personality that features a strange mix of humility and bravura, Green is a commanding presence, more eager socializer than the introverted observer associated with his profession. Though he's a novelist, he clearly understands the power of the moving image — and how to leverage his celebrity.

The Florida native, who now lives in Indiana with his wife and two young children, wrote his first young adult book, "Looking for Alaska," in 2005 and followed that with two more novels, "An Abundance of Katherines" in 2006 and "Paper Towns" in 2008. But it was the combination of the YouTube video blog he's nurtured with his brother Hank Green for the last seven years — and his adept advanced marketing for "Fault," which included editing the first chapter at a live event — that launched him into the stratosphere. 

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Green now gets stopped on the street by adoring fans, primarily teenage girls who have embraced his online movement dedicated "to raising nerdy to the power of awesome." On both Twitter and YouTube, he has more than 1 million followers (dubbed "The Nerdfighters"). And he and his brother sold out a live variety show at Carnegie Hall in January 2013 celebrating the one-year anniversary of "Fault."

Green has been keeping his fan base engaged throughout the filming, using Twitter, Instagram and his YouTube channel to show off behind-the-scenes high jinks with the young cast that includes Shailene Woodley, Ansel Elgort and Nat Wolff.

"I think he has a real drive as not only an author but as a businessman and as a brand manager," said "Fault" producer Wyck Godfrey, who has worked with celebrity authors including Stephenie Meyer and Nicholas Sparks. "He understands now that he's in a position to influence culture and he's going to use every avenue he can take to do that. It's not only smart as a businessman, but it's admirable in that the message he's putting out there is so positive."

An inspirational friendship

A religious studies and English major in college, Green always harbored aspirations of becoming a novelist, but his practical nature led him to pursue a "day job" as an Episcopal priest. A seven-month stint as a chaplain in a children's hospital, however, convinced him he wasn't cut out for the ministry.

Green wrote his first young-adult novel, "Looking for Alaska" (about his time in boarding school), while working for the book review journal Booklist but kept returning to a semi-autobiographical, in-progress piece about a chaplain in a children's hospital. "It was brutally bad," he said.

It wasn't until he met 14-year-old cancer patient Esther Earl at a Harry Potter convention that his thoughts about children with illness took shape as a real story. He spent a lot of time with Esther toward the end of her life, organizing her Make-a-Wish (bringing all her online friends to Boston for a weeklong celebration). After she died, his novel came alive. 

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He says flatly that Esther was not the basis for his protagonist, Hazel Grace, who has stage 4 thyroid cancer.

"Hazel and Esther are very different people, but I could never have written Hazel had I never been friends with Esther," said Green, sitting outside a hotel in downtown Pittsburgh, dressed casually in a green "Hunger Games"-themed T-shirt and jeans. "My friendship with Esther taught me two things: how empathetic and outwardly focused teenagers can be, and that Esther's life was still a good life and she was glad to have lived it."

Green's personal relationship with "Fault," coupled with his previous unsatisfying experiences selling his books to Hollywood, made him reticent to option his latest work. (His first novel is in development at Paramount Pictures, and rights to the other two have reverted to him after the projects failed to take off.) He wouldn't even let his publisher, Penguin Group, issue advance copies of the "Fault" manuscript to interested filmmakers ahead of the book's 2012 release. 

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