On Thursday night, Nina Dobrev will appear in her last episode of “The Vampire Diaries,” the hit CW show she has starred in for six years. To a lot of people, this is a very big deal.

Then, just as many people have no idea what that means. Who cares if one actress is leaving a show?

Let’s put it this way: Nina Dobrev leaving “The Vampire Diaries” is like if Bones left “Bones.” If House left “House.” If Mindy Kaling left “The Mindy Project.”

In other words, it’s inconceivable that an actress that central to a show is leaving before its conclusion: “The Vampire Diaries” has been renewed for a seventh season and will continue without her. The show, which scored the biggest CW premiere in history, with nearly 5 million viewers in 2009, now averages about 3 million viewers a week: But in addition to traditional viewing habits, it has a rabid online fan base that is almost impossible to quantify.

Still, to truly grasp the extreme significance of “The Vampire Diaries,” a series deeply consumed by its young fans and readers of the original book series, we had to ask someone embedded in the culture. We turned to Carina Adly MacKenzie, a writer on “Vampire Diaries” spin-off “The Originals.” She’s also a former reporter who used to cover “The Vampire Diaries” for the Los Angeles Times and Web site Zap2It. 

To partly explain why the show caught on, it launched right around the explosion of the massively popular “Twilight” phenomenon, when the movies and vampires in general were inescapable in pop culture.

“After what ‘Twilight’ meant to people, [‘The Vampire Diaries’] became this obsession,” MacKenzie said. “A lot of why people became attached were the three characters in the center of the show.”

Those three characters include Dobrev’s Elena, the protagonist originally caught in a wildly intense love triangle with two vampire brothers, Damon (Ian Somerhalder) and Stefan (Paul Wesley). She was a human, but then turned into a vampire, and turned back into a human. It’s complicated. Don’t even ask about the doppelgangers that meant Dobrev wound up playing multiple characters over time.

Anyway, MacKenzie speculates, viewers were drawn to Elena because she was an “every girl.” Her character went through a lot of tragedy (her parents were killed in a car accident, not to mention the deaths of other loved ones), and for all her troubles, she was rewarded with two dark, extremely handsome mysterious strangers, Stefan and Damon. She eventually chose Damon as her soul mate; fitting, because Dobrev and Somerhalder dated for three years in real life and became a tabloid-favorite couple.

Watching the show every week, younger viewers got especially attached, devoting Tumblrs and Twitter feeds to the characters and becoming especially vocal about what they liked and didn’t like.

“When I was covering the show, I used to compare it to religion in the way that people are so fervent and passionate about various couples,” MacKenzie said. “I don’t think something that a casual TV viewer can relate to…it is consuming.”

As an example, every week, you can reliably count on at least one phrase relating to the show popping up in the Twitter worldwide trending topics. Like these, a day after Dobrev announced her departure:

And, yes, Dobrev announced in advance that she would be departing: No surprise deaths or exits in this show. Naturally, some fans had trouble accepting the news she was leaving.

“People were pretty distraught. I imagine that’s why they announced it early instead of waiting for the show to air,” MacKenzie said. “My guess is that was what the thought process was: Give people time to let it sink in and process, and enjoy the final few episodes and not feel like they were steamrolled.”

So why would Dobrev walk away from a) a cash cow and b) a show whose viewers are so devoted? Dobrev cited the long hours on the show, and after six seasons, she’s ready to move on to movies.

Of course, there are some conspiracy theorists that assume it’s because she no longer wants to work with Somerhalder, her ex-boyfriend, who recently got married to “Twilight” star Nikki Reed. Dobrev and Somerhalder split in May 2013 though, by all accounts, remain friendly. They even showed up to a possibly contractually obligated appearance to accept the “favorite on-screen chemistry” trophy at the People’s Choice Awards after their breakup, joking about how awkward it was to be love interests on the show.

“Sources” emphasized to E! Online that seeing her ex get married had nothing to do with why Dobrev left, adding that she always planned to stay for six seasons (the standard length of a TV contract) and attempt a movie career. Driving that point home, Dobrev posted this tweet on the day of Somerhalder’s wedding:

In any case, expect there to be a lot of buzz around Dobrev’s departure on Thursday: Viewers get their last glimpse at Elena, who may have died at the end of the penultimate episode. (Expect lots of flashbacks.) In a conference call announcing the CW’s new fall schedule, President Mark Pedowitz said that the show has been building up to a re-tooling all season. MacKenzie also points out that after Season 4, it was still a romance-based show but that it was more about the ensemble than the love triangle.

It went from being all about Elena and who is she going to choose to about a group of people, family and friends, and the love triangle faded into the background,” MacKenzie said. “I do think that fans are going to be surprised by how much they’re still going to love the show without it.”

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