Public Hater, Private Fan

A character spends a significant amount of time telling another character that their favorite in-universe fictional franchise is completely idiotic and cites examples. At no point does it ever occur to wonder how exactly the detractor is so well informed.

Then they make a verbal gaff, making a reference only a fan would know, revealing that despite professing to hate the franchise, they were a fan all along. Usually followed by a lame attempt to pass off their knowledge as casual interest. Of course, nobody believes them.

Compare the Obsessed Hater subset of Hate Dumb, which may sound a lot like this.
Contrast Closet Geek where the fan simply hides their interest as opposed to pretending to hate the franchise.

Examples

Anime and Manga
  • In Eromanga Sensei, Masamune Izumi is a light novel writer with problems to get success with his novels, when he knows Elf Yamada, a more successsful and famous light novelist who rivals Masamune and even he hates in some moments. After passing some time writing together, Masamune admits he's fan of Elf's novels and even they were an inspiration to be a light novelist like her.

Live-Action TV
  • Hunter Street: Sal spends all episode saying how unrealistic McMacho is and challenges Max to prove it by having each of them construct two traps to find out which one is superior. When Max quotes a line from a McMacho episode after his trap is effective, Sal instantly cites the episode name and number that the quote originated from. Then he tries to claim he only watched a few episodes.

Music
  • In The '70s, it was de rigeur for anyone wanting to be seen as having street cred to despise pop music and "froth" of the Abba variety whilst expressing enthusiasm for Punk Rock in all its forms. Prog Rock also suffered from the same "either-or" dichotomy. Yet many fans and practitioners of punk and New Wave have since revealed that they actually liked and respected Abba and the despised hippy old timer bands for qualities like showmanship, musical and lyrical competence, and — yes — for some nice tunes. They just weren't able to say so in public, and those acts remained a guilty private pleasure.

Webcomics
  • Played with in Unwinder's Tall Comics: "One of these characters is a brony". Unwinder talks about how he doesn't understand the My Little Pony periphery fandom at all, and his friend Barbecue Sauce agrees with everything he says. But Sauce has MLP dolls in his room, which starts playing with as soon as Unwinder leaves... And then it's All Just a Dream. In the waking world, Unwinder is a very outspoken MLP fan, while Barbecue sauce doesn't like it at all.

Western Animation
  • Arthur gets addicted to The Love Ducks. He secretly pretends to hate it while watching it instead of the much more mature "Dark Bunny" cartoon. At the end of the episode, Francine turns out to have also been a fan all along (despite leading the mockery when Arthur was caught watching it).
  • Beast Wars: For all of Dinobot's bluster about Predacon virtues, he actually holds a great deal of respect for the Maximals. Largely due to Dinobot being an honorable warrior, while the remaining Predacons are not so honorable. Which the Maximals are.
  • Fairly OddParents: Trixie reveals she secretly loves comic book superheroes... but publicly claims they're childish and un-girly.
  • Gravity Falls: Dipper denies being a fan of musical group BABBA, at least until he learns that the Multi-bear has been rejected by the Manotaurs for being a fan of bubble-gum pop music.
  • Scooby-Doo! Mystery Incorporated: In "The Grasp of the Gnome", a man who says he hates pirates is shown to be dressing and acting like one when nobody's around.
http://tvtropes.org/pmwiki/pmwiki.php/Main/PublicHaterPrivateFan