Posters for Nickelodeon are seen in the video game room. Natt Wolf had starred in The Naked Brothers Band, a Nickelodeon show. He's also singing lines from a song he and his brother Alex wrote during that scene.
Beam Me Up, Scotty!: Augustus never claims to feel kinship with Anne Frank because she died "of an illness" like his own. He and Hazel both clearly feel kinship with her because she died young, but the alleged murder-denying quote never occurs. That is a reworking of a much worse quote from an interview with John Green, who says that Anne Frank is 'demystified' because she 'just died of an illness'. Yeah, an illness that she contracted in a concentration camp.
Breakaway Pop Hit: "Boom Clap" by Charli XCX. While the movie was hugely successful, the song is fast distancing itself from its origins as of this writing.
Dawson Casting: Shailene Woodley was twenty three playing Hazel at sixteen. Ansel Elgort was borderline, being twenty and playing an eighteen-year-old (and amusingly is listed as nineteen at one point by Isaac).
Shailene Woodley had long blonde Rapunzel Hair, which she cut short to play Hazel. This affected The Divergent Series: Insurgent, where Tris cuts her hair drastically shorter than she does in the book - as Woodley didn't have time to grow it out again.
Lily Kenna, who played the young Hazel, had her hair shaved off on-screen.
Method Acting: Shailene Woodley learned how to shorten her breath to properly simulate how Hazel would speak in real life.
Shrug of God: John Green has had to do a lot of this to fans who want to know what happened to the characters before/after the contents of the book; this becomes somewhat humorous in light of the fact that there’s also a significant in-universe example where Hazel and Augustus ask Van Houten what happened to the characters of An Imperial Affliction and he tells them he doesn’t know.
The original ending had Hazel and Van Houten try to find a way to honor Gus's death by dying themselves in a way that would accomplish something and was meaningful to his life. They plan to go out and kill a drug lord and die at the hands of his bodyguards, but apparently decide against the idea before the story's conclusion. One editor, upon reading this ending, remarked, "I can't tell if the last 40 pages are a joke." The again, neither can John Green himself.
Originally, the story was going to be from Isaac's point of view.
The scenes in Amsterdam were intended to be shot on location. However, filmmakers were intimidated by the poor October weather - and also denied permission to film inside the Anne Frank House. As a result, a replica was built, and Pennsylvania doubled for Amsterdam.
Mae Whitman was John Green's preferred actress to play Hazel. Nonetheless, he was blown away by Shailene Woodley's audition and gave her the Approval of God.
A scene was filmed featuring an argument between Gus and his mother right before the trip to Amsterdam, that also features in the book. It would have acted as Foreshadowing that Gus's cancer has come back.
Write Who You Know: Hazel is based on a girl John Green met at a Harry Potter convention, when he noticed her carrying an oxygen tank. He came to like her through various vlogs she put online, and they exchanged emails - much like Hazel and Gus do with Peter. The girl's middle name was Grace, like Hazel's. They remained good friends until her death in 2010.