[Warning: This article contains spoilers for The Fault In Our Stars.]

Every morning, they come: girls in Doc Martens boots and head scarves and bright pink hair, floral backpacks and hooded sweatshirts and Converse. They have figured out where the shoot's locations are going to be via Twitter and Instagram, and they arrive on set in groups of two or three or more, radiating a sense of uncomplicated teenage girl-ness that is infectious, really — the way they stand off to the side with hopeful smiles on their faces on rainy Amsterdam streets, umbrellas in hand, quiet when shushed by a production assistant, necks craning to catch just a glimpse of their hero. Occasionally there is one, like the Emma Stone doppelgänger in front of the American Hotel on the Leideskade, who can't contain herself and, upon spotting him, stares and stares until finally she begins inching toward where he is standing by the monitors, under a tarp, and is then shooed back by security.

You might assume that the girls are here on these unseasonably cold October days for the film's two beautiful young stars, Shailene Woodley and Ansel Elgort, who are agreeable about posing for photos and talking to people in the way that actors on the cusp of huge stardom tend to be. But they are really there for John Green, the unassuming, boyishly handsome 36-year-old author of The Fault in Our Stars, his best-selling 2012 book about two teenagers with cancer who fall in love. Green has written best-sellers before, but Fault is his first book to get made into a film. And in an unprecedented move, the studio, Fox 2000, has allowed — encouraged, in fact — Green to be present for and even document the entire four-month shoot, first in Pittsburgh (a stand-in for Indianapolis, where the book takes place and where Green lives with his wife, Sarah, and their two children), and now, at the very end of the shoot, for four days in Amsterdam. But whereas in Pittsburgh he says he had been left pretty much well alone, Amsterdam is a different story. Here, he is the main event.

"We're fangirls," says a 15-year-old Amsterdam girl named Zita, who has read Green's novels and follows him on YouTube and Twitter. She sighs. "He's just...yeah."

Another Dutch teenager named Daphne traveled an hour by train to come to the set. "We just got here — we couldn't find it," she says. "We were on the other side of the Runstraat. I haven't even seen him. Oh! I see him!" She points, and her two friends say things excitedly to each other in Dutch.

"He's been taking pictures with his fans," I tell her.

"Oh my god. I was here yesterday with my dad to wait for him and I want to see him," she says.

I ask if there were any other authors she'd want to come out in the rain and wait for.

"Yeah, but she's dead," says Daphne.