When one character in a pair who'd otherwise be different and/or unrelated discovers that they actually have some things in common, more than the other character would want to believe.
This is usually the resolution of an Enemy Mine or Locked in a Freezer plot: two enemies learn that there is more that unites them than divides them. The punch line to about 50% of plots in an Odd Couple or Odd Friendship series.
On the darker side, this trope can also be used to lampshade the fact that our hero is really just a hair's breadth away from villainy. In these cases, it's almost always the villain who is the first to notice: our hero has him cornered and even seems to relish the prospect of finally ridding himself of his nemesis, and the villain deftly points out, "We're not so different after all", commonly as a way to guilt-trip the hero. This may also be used to try to sway the hero to join his side. The hero realizes how close he is to crossing the line, and spares the villain (though every once in a great while, he'll decide that it's worth crossing the line, off the villain anyway, and then have several episodes of moral anguish over the darkness in his soul: see Shoot the Dog). Especially common when the villain is the hero's Evil Counterpart.
Also a bad thing when the bad guy points it out after the hero has bested him through questionable tactics. Often followed by the hero's hysterical protests along the lines of, "I'm not like you! I'll never be like you!" Occasionally followed by the hero's observation that they are different, because of some other aspect that the opponent overlooked or intentionally ignored. However, the hero sometimes has some snappier comeback, like "You noticed?" or "That's why I can beat you," and worries about the differences later—or a comment that the dog that protects the flock is a very close relative of the wolf that ravages it. Then again, a simple "shut up" can work wonders. Another common rebuttal is for the hero to acknowledge that they share some similarities, but are sufficiently different where it counts; not all similarities are equal.
The Horseshoe Effect is when two people claim to be ideologically opposed to one another, but nevertheless have many beliefs in common.
When the villain who believes this also believes that the hero denies it because he is a Slave to PR, he often sets up What You Are in the Dark to get the hero to act like it. The failure rate for this approach is... high.
Heroes sometimes use this to confront a Well-Intentioned Extremist or a Knight Templar as to call them out or to try to redeem them. Being compared to their enemy will often trigger their Berserk Button. Contrasts with Not So Similar and You Keep Telling Yourself That. Also compare Shared Family Quirks, which is when two seemingly different family members have something in common behavior wise. Hitler Ate Sugar is when this trope is stretched to the point of becoming a fallacy.
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- A common 3rd Reich joke was comparing Goebbels to a Jew (from Nazi propaganda, mind it): He didn't look the least "arisch" (with the crippled leg and all), he had a sharp but vile intelligence, he was a renowned horndog, and to top it he had a Jewish first name.
- Nikita Krutschev is giving a speech on the necessity of de-stalinizing the country. Someone in the crowd yells "And why didn't you do anything about it before Stalin died?!" Krutschev stands straight up and bellows "WHO SAID THAT?!?", looking left and right. When there is absolute silence and no one owns up to it, he says (in a normal voice) "That is why".
- Jimmy Jacobs has repeatedly tried to pull this on Jon Moxley. This once lead to the two being chained together in Insanity Pro Wrestling, for a dog collar match after Jacobs cost Moxley the promotion's top championship belt and said that when he looked at Moxley he stared at himself in a mirror.
- Raven, after deciding that he was insane while in Impact Wrestling and could not be cured, decided that Abyss was the same way and to make Abyss admit it.
- Bray Wyatt said this to Dean Ambrose on the 10/27/14 episode of WWE Raw. Wyatt said that looking at Ambrose was like seeing "a deranged reflection" of himself.
- Kevin Kelly frequently does this, where appropriate, when addressing feuding members of the Ring of Honor roster. For example, when Kenny King returned from TNA and went on a rant about why he couldn't stand ACH, Kevin asked if The All Night Express, King's Tag Team, weren't a pair of ACHs a few years ago.
- Mentioned by name in Hallowicked's promo before his CHIKARA Grand Championship title defense against Dasher Hatfield at CHIKARA The Immaculate Election, July 26, 2015. He said that Dasher has a "certain nobility about him" that neither he nor Nazmaldun can understand, but that when the Grand Championship is on the line he wants to see how far Dasher is willing to go. At Babylon Springs on November 8th, The Nightmare Warriors (Hallowicked and Frightmare) defeated The Throwbacks (Dasher and "Mr. Touchdown" Mark Angelosetti), The Osirian Portal and Hermit Crab and Cajun Crawdad in a four-way tag after Dasher abandoned the match when a piece of metal fell out of the elbow pad he'd been using, proving Angelosetti was still cheating. Dasher surrendered the points Dasher's Dugout (Throwbacks/Icarus/Heidi Lovelace) had accumulated in the Challenge of the Immortals tournament, confirming his nobility. However, his shocking heel turn as Grand Champion proved Wicked right, even though his turn was pure selfishness, as opposed to being put under a spell like Wicked was.note
- Big Finish Doctor Who: In the lead-up to the 40th anniversary, a trilogy of audio dramas was released, "Omega", "Davros", and "Master", to show how the Doctor and these villains were not so different. This is really emphasized in "Master", with The Reveal that the Doctor made a deal with Death while a child so his memories of a murder he committed would go to his friend, which turned the Master evil. Of course, the show gives us the real story of what makes the Master who he is and it's quite different.
- How Green Was My Cactus: "Boris and Gorby go to McDonald's" is about Mikhail Gorbachev and Boris Yeltsin opening up to western capitalism. Their experience with fast food gives them the following revelation:
"Is not so different! We stand for ages in queue to get served by pimply little people who work like hell for rotten veggies, and the food is terrible!"