Who Are You and What Did You Do to X?

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

Occasionally even happens to an actual impostor.

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

A subtrope of OOC Is Serious Business.

Anime and Manga
"Genma: I carved this on our honeymoon. A memory of love. Ranma: Who are you and what have you done with my father?"
 * Ranma ½: At one point, Genma talks about him and Nodoka being Happily Married. Ranma doesn't believe it though.

Fan Works
"FROM Azu-nyan <3 TO Yui-senpai
 * Frequently used in Harry Potter fanfics whenever Hermione Granger does anything that defies the Flanderized "homework martinet" characterization she gained in earlier fics.
 * In the Undocumented Features story The Order of the Rose "The Federation Lives Forever!" there is a moment when notoriously slothful guitarist Yui Hirasawa of K-On! texts her much more industrious kohai Azusa Nakano one Saturday afternoon and invites her to come over and practice. Azusa promptly texts back:

Who are you and how did you get Yui-senpai's phone?"

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:

Music

 * There's a filk song on this — Who Are You And What Have You Done With...

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
"Ennesby: Where's Tagon, and what have you done with him?"
 * Vexxarr with Minionbot and Vexxarr himself. "We need to make sure you're really you..." And Vexxarr again.
 * Schlock Mercenary had [//www.schlockmercenary.com/2010-06-27 this one]:


 * And "[//www.schlockmercenary.com/2018-04-30 May I speak to the real Captain Landon?]" (minor spoilers).
 * Girl Genius, when Dupree corners Gil about the whole "acting out of character too much" thing (we already know he's under mind control, and some of other people dealing with him know or strongly suspect this).
 * And in a less serious way, Martellus.
 * Questionable Content with [//questionablecontent.net/view.php?comic=430 the real Pintsize], and [//questionablecontent.net/view.php?comic=4047 the real Hannelore] (who used to be almost terminally germophobic and after a while improved to moderately germophobic, so it is out of character).
 * Flaky Pastry here. Which is not entirely unreasonable, since that's what caused the sudden change.

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