“The fault, dear Brutus, is not in our stars / But in ourselves …”

So says Cassius in Shakespeare’s “Julius Caesar,” making the point – en route to an assassination – that choice rather than fate is what drives the destiny of mankind.

Which is precisely the opposite of what John Green meant when he titled his novel “The Fault in Our Stars,” whose movie adaptation opens Friday.

“To be fair, I’m not dissing on Shakespeare,” said Green, whose career as a young-adult novelist has been such stuff as dreams are made of. “But that line is often taken out of context. It’s one thing to say it if you’re a Roman nobleman and have been born into a life of privilege. But very often the quote is used in a general way to say that people make their own fate. And that’s ludicrous.”

The fate of Green’s characters is cancer. Hazel Grace Lancaster and Augustus Waters, played by Shailene Woodley and Ansel Elgort, both have it, she in the lungs, he in the leg. That their destinies become intertwined makes for a most unusual love story, one that has attracted a fan base both devoted and huge: When the cast did a press tour recently of midsize U.S. cities, attendance far exceeded expectations, with 2,000 showing up in Nashville, 3,500 in Cleveland, 3,000 in Dallas and 5,000 in Miami.

“It blew everyone’s expectations,” said Elgort, whose previous film was “Divergent,” also alongside Woodley. “And it’s big, really big, because the story makes people think. And it’s even bigger than the story.” He said when the cast appeared in Dallas, a girl came onstage and she spoke about her own cancer.

“After she asked her question,” Elgort said, “the whole crowd started shouting, ‘Caroline! Caroline! Caroline!’ And she started crying. I think Nat Wolff started crying. It was a really beautiful moment. It’s bigger than us.”

Elgort’s leading lady agreed. “I think with these film adaptations you get these really obsessed crowds, but they’re really obsessed with seeing or being around somebody famous,” Woodley said. “But with this, it never felt like they were there to see us. They were there to celebrate the story. It was very profound, and I wasn’t expecting it.”

“The Fault in Our Stars” is a love story whose subtext is its heroes’ fragile mortality. As such, it appeals to the melodrama junkie in all of us. “It’s the same way young people get attracted to ‘Romeo and Juliet,’ or ‘Titanic,’ ” director Josh Boone said. When you’re young, he said, love is a life-or-death proposition; in Green’s story, that’s literally the case.

How Green’s young-adult fans know about his books (which include “Looking for Alaska” and “An Abundance of Katherines”) can be chalked up partly to the VlogBrothers video blog that he and his brother Hank maintain on YouTube, a kind of unformatted conversation whose followers are known as Nerdfighters. “You don’t have to be a good video editor to be a successful author, but it’s so helpful to have a supportive community of readers,” said John Green, who offered that part of the appeal of “Fault in Our Stars” is a lack of irony.

“I think young people like to ask questions about the meaning of life and the meaning of suffering,” he said. “You don’t have to use irony in your approach to meaning. They’re un-ironically interested.”

Considering the fan fervor, there was a concern among everyone about getting the movie right, but not to the point of distraction.

“I couldn’t think about that when we made the movie,” Boone said. “I had to focus on the script and making sure John was happy. And he made us feel like we were getting it right.”

On a more basic level, fans will be interested in the romance, and the chemistry between Elgort and Woodley, who made her big splash alongside George Clooney in “The Descendants” (2011), and whom Boone knew was interested in the role. He went ahead and auditioned everyone else, anyway.

“I thought she was great, but I hadn’t met her, and based on what I’d seen and her pictures, she seemed so strong and athletic,” Boone said. “Eventually, she came in and blew everybody away. I said, ‘Why did I make it so hard on myself?’ ”

None of this changed the fact that he needed an actress to play a young woman suffering from terminal lung cancer. Part of the solution was found in Elgort, who’s 6-foot-4, towering over Woodley, and naturally making her seem more delicate by contrast.

“Completely,” the actress agreed. “When he came to audition, I was just hanging there with my big, baggy jeans, and I’m 5-9, so I’m kind of tall, and it’s very rare to find really tall actors. I would not have looked as fragile with someone shorter. And there is something so beautiful about having to tilt my entire chin up just to look at his face. It all worked in our favor. We got lucky.”