Who Are You and What Did You Do to X?

ck phrase

A stock phrase to invoke and/or parody "Spot the Imposter" game, that is, happens when someone seems to act out of character. For comedy purpose, usually it's said when the "impostor" performs better.

Occasionally even happens to an actual impostor.

Seen It a Million Times.

Web Comics

 * Vexxarr with Minionbot and Vexxarr himself.

Real Life

 * A common phase related to this trope is "Who died (or left) and put you in change".