"It's funny. Every crook I meet wants to tell me how much I'm just like them."We're not so different, you and I. In another time, another place, I might have called you friend! Two characters who seem radically different turn out to have more in common than they would like to believe. As a good thing, the frequent 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. As a bad thing, the realization 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." 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 might be for the hero to acknowledge that they share some similarities, but are sufficiently different where it counts; not all similarities are equal. 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. Please note when heroes use this to confront a Well-Intentioned Extremist or a Knight Templar as to call them out. Being compared to their enemy will often trigger their Berserk Button.
— Edward Elric to Psiren, Fullmetal Alchemist