When a pair or groups of characters — who'd otherwise be different and/or unrelated — discover that they actually have some things in common, after all.
This is usually the resolution of an Enemy Mine or Locked in a Freezer plot: two clashing individuals learn that there is more that unites them than divides them.
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 specific 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.
In lighter cases not involving a hero and villain situation, expect the other side to protest strongly with something like, "I'm not like you! I'll never be like you!" when the person they are feuding with points it out. Another common rebuttal is when one side acknowledges that they do share some similarities with the other, but are sufficiently different where it matters.
Please note: for this trope to apply the similarity between the two or more opposing characters needs to be acknowledged in-universe either by the characters realizing it themselves or by a third party. It isn't limited to only two characters and those involved don't necessarily have to be sworn enemies to qualify; any clashing characters count as long as their similarities are explicitly mentioned, hence the "remark" part. Mere implied, subtextual similarities, no matter how obvious they are, or observations made by the audience are NOT examples unless they are also lampshaded within the work itself.
Closely related to Foil, which is a literary term that describes a character who highlights and accentuates the traits of another character by contrast, and Mirror Character, which describes characters highlighted by similarities. However, in these cases, lampshading isn't required.
The Horseshoe Effect is when two people claim to be ideologically opposed to one another, but nevertheless have many beliefs in common.
Contrast with Not So Similar. Also compare Shared Family Quirks (when two seemingly different family members have something in common behavior-wise), Hitler Ate Sugar (when this trope is stretched to the point of becoming a fallacy) and Too Much Alike (when two characters don't get along because of their similarities). See also Mirroring Factions.
