Green wanted to write “an unsentimental cancer novel” that offered “some basis for hope.” Illustration by Bartosz Kosowski

In late 2006, the writer John Green came up with the idea of communicating with his brother, Hank, for a year solely through videos posted to YouTube. The project wasn’t quite as extreme as it sounds. John, who was then twenty-nine, and Hank, who was three years younger, saw each other about once a year, at their parents’ house, and they typically went several years between phone calls. They communicated mainly through instant messaging.

Hank was living in Missoula, where he’d started a Web site about green technology. John was living on the Upper West Side while his wife, Sarah Urist Green, completed a graduate degree in art history at Columbia. He had published two young-adult novels, “Looking for Alaska,” in 2005, and “An Abundance of Katherines,” in 2006, and was working on a third. Like the best realistic Y.A. books, and like “The Catcher in the Rye”—a novel that today would almost certainly be marketed as Y.A.—Green’s books were narrated in a clever, confiding voice. His protagonists were sweetly intellectual teen-age boys smitten with complicated, charismatic girls. Although the books were funny, their story lines propelled by spontaneous road trips and outrageous pranks, they displayed a youthfully insatiable appetite for big questions: What is an honorable life? How do we wrest meaning from the unexpected death of someone close to us? What do we do when we realize that we’re not as special as we thought we were?

Green was more forgiving toward adults than Salinger was, but he shared Salinger’s conviction that they underestimate the emotional depth of adolescents. Green told me, “I love the intensity teen-agers bring not just to first love but also to the first time you’re grappling with grief, at least as a sovereign being—the first time you’re taking on why people suffer and whether there’s meaning in life, and whether meaning is constructed or derived. Teen-agers feel that what you conclude about those questions is going to matter. And they’re dead right. It matters for adults, too, but we’ve almost taken too much power away from ourselves. We don’t acknowledge on a daily basis how much it matters.”

Y.A. novels are peculiarly well suited to consideration of ethical matters. It seems natural when a high schooler like Miles Halter, of “Looking for Alaska,” is depicted struggling to write essays on topics like “What is the most important question human beings must answer?” Miles is equally preoccupied with girls and with collecting the dying words of famous people. (His favorite: Rabelais’s “I go to seek a Great Perhaps.”) Though “Looking for Alaska” sold modestly, it won the Michael L. Printz Award, the American Library Association’s honor for best Y.A. book of the year. At the time, Green was living in Chicago, working at the association’s magazine, Booklist, where he had reviewed books in a peculiar constellation of subjects: conjoined twins, boxing, and theology. Upon graduating from Kenyon College, in 2000, Green had thought of going to divinity school, and he worked for six months as an apprentice chaplain at a children’s hospital in Columbus. He found the experience almost too sad to bear, and decided that such a life was not for him. Still, he remained deeply interested in spiritual matters, with one exception: “Is there a God?” struck him as “one of the least interesting questions.”

After “Alaska” won the prize, Green quit his day job. He got more writing done, but he missed the intellectual camaraderie that he’d always had with his peers. The YouTube project was, in part, an attempt to fill that void. (It was also a smart marketing stunt, though Green could not have predicted how smart.) Hank had reservations about becoming the repository for John’s excess energy. He told me, “I found John exciting and smart and interesting but also a little dramatic. He gets frustrated easily. He’s anxious. Hypochondriacal.” At the same time, he said, “John, for me, has always been the baseline of what was cool and valuable and important. If he liked a band, I’d buy all of their CDs and memorize them and become a bigger fan than he ever was.”

In 2006, YouTube was entering its second year, and people were starting to post video diaries, which, in their more theatrical moments, looked like performance art staged in somebody’s basement. John Green was a fan of several such series, especially “The Show with Zefrank,” which enlisted viewers in quirky projects, such as dressing up their vacuum cleaners as people. Hank shared John’s enthusiasm for these experiments, and it trumped any hesitations that he had. “We really believed in the importance of online video as a cultural form,” Hank said.

The Greens started posting videos several times a week, under the name the Vlogbrothers. The project was less a conversation than an extended form of parallel play. They shared personal stories—John confessed that the only sports trophy he ever got was made by his parents, and bore the inscription “All-Star in Our Hearts”—but mainly they exchanged ideas. The brothers had signature preoccupations, which they discussed with excitable urgency, talking into the camera at tremendous speed. John discussed books, existential anxiety, and pizza; Hank was into science, math, and corn dogs. John invented a highly undignified “happy dance”; Hank wrote and performed songs, many of them about Harry Potter. The tone of their monologues ranged from goofily informative (how giraffes have sex) to wonkish (“Why Are American Health-Care Costs So High?”). Many posts dispensed adult wisdom, but in a reassuringly modern way. In a post advising boys on how to charm a girl, John jokingly said, “Become a puppy. A kitten would also be acceptable or, possibly, a sneezy panda”—an allusion to a popular clip on YouTube. But he also said, “If you can, see girls as, like, people, instead of pathways to kissing and/or salvation.”

The Greens’ vlogs were filled with in-jokes and code words that rewarded dedicated viewing. “D.F.T.B.A.” stood for “Don’t Forget to Be Awesome,” and John referred to his wife as “the Yeti,” because she was much talked about but—by her choice—never seen on camera. When a brother broke a rule that they’d established, such as posting a video longer than four minutes, the other brother could impose a punishment. Hank once had to spend fifteen consecutive hours in a Target; John had to eat a generous helping of “slobber carrots.” (His toddler, Henry, provided the slobber.)

In February, 2007, John was stuck at the Savannah airport, and he spotted an arcade game called Aero Fighters. He initially misread the name as “Nerdfighters,” and later, in a video, he started riffing: what if Nerdfighters were a real game? As he put it, “The band geek would be, like, ‘I will destroy your ears with my tuba!’ And the theatre guy would be, like, ‘I am an expert at sword fighting!’ And the English nerd would be, like, ‘Hmm, I know a lot of Shakespeare quotes!’ ” Why did people still pick on nerds, anyway? Who did the popular guys have on their side—George W. Bush and Tom Brady? Green declared, “I raise you an Abraham Lincoln and a Franklin Delano Roosevelt and . . . an Isaac Newton, a William Shakespeare, a Blaise Pascal, an Albert Einstein, an Immanuel Kant, an Aristotle, a Jane Austen, a Bill Gates, a Mahatma Gandhi, a Nelson Mandela, and all four Beatles. We win.”