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.

Examples of Who Are You and What Did You Do to X? include:

Anime and Manga

Ranma ½: At one point, Genma talks about him and Nodoka being Happily Married. Ranma doesn't believe it though.

Genma: I carved this on our honeymoon. A memory of love.
Ranma: Who are you and what have you done with my father?

Fan Works

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:

FROM Azu-nyan <3
TO Yui-senpai

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

When Akane apologizes to Ranma for the first time in the Ranma ½ fic series Tales of Ranma and Ranko, he asks her, "Are you really Akane?"

Film

From the 1997 Fran Drescher film The Beautician and the Beast:

Joy Miller: Who are you and what did you do with the little Nazi kid?

In Harry Potter and the Order of the Phoenix, Hermione enthuses about how breaking the rules unexpectedly felt good, to which Ron responds "Who are you and what have you done with Hermione?"

Live-Action TV

From the Gossip Girl episode "Hi, Society":

Nate: Who are you and what did you do with Blair Waldorf?

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

Vexxarr with Minionbot and Vexxarr himself. "We need to make sure you're really you..." And Vexxarr again.
Schlock Mercenary had this one:

Ennesby: In a TUX? Where's Tagon, and what have you done with him?

- And "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 the real Pintsize, and 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.