Who Are You and What Did You Do to X?

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.

Occasionally, this phrase can be used as a playful compliment, when a character has undergone a makeover, either minor or radical.

Seen It a Million Times.

Fanfiction

 * Frequently used in Harry Potter fanfics whenever Hermione Granger does anything that defies the Flanderized "homework martinet" characterization she gained in earlier fics.

Film - Live Action
"Joy Miller: Who are you and what did you do with the little Nazi kid?"
 * From the 1997 Fran Drescher film The Beautician and the Beast:

Periodicals

 * Who are you, and what did you do with my spouse?, an article in the March 2013 issue of the American Psychological Association's gradPsych magazine.

Web Comics

 * Vexxarr with Minionbot and Vexxarr himself. "We need to make sure you're really you..."

Television
"Nate: Who are you and what did you do with Blair Waldorf?"
 * From the Gossip Girl episode "Hi, Society":

Real Life

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