We Would Have Told You But

An episode in which a main character and the audience is kept in the dark by their friends or colleagues. This is to pull off a Sting against someone else, never a main character.

They will be given the excuse at the end ... "We would have told you, but we needed your reactions to look genuine." In settings that include telepathy it's often done to foil even that.

It's a Not Himself with the numbers inverted.

Sometimes followed by No Hard Feelings.

When this is done to the actors, it is Enforced Method Acting.

Comic Books

 * In one Black Canary miniseries, when Black Canary's adopted daughter Sin is kidnapped, Green Arrow makes it appear that the rescue attempt, badly bungled, killed Sin. He knows the kidnappers will watch Black Canary and note if she does not show grief, so he keeps her in the dark to make sure her reaction will be authentic, and he does not expect her to forgive him when he reveals the truth.
 * In one issue of Impulse, Max and The Trickster pull off a plot like this to take down a pair of mob bosses.

Film

 * In the movie Chicago, Billy Flynn doesn't tell Roxie that he  in order to keep her from screwing up his plan during her trial.
 * In The Dark Knight,, but the reveal happens so quickly and in the middle of the film's biggest escalation scene that you might miss his reasoning on the first viewing.

Literature

 * Sherlock Holmes pulls this one on Watson in "The Adventure of the Dying Detective," making it Older Than Radio.
 * In Bert Coules's radio adaptation of "The Dying Detective," Watson fails to accept Holmes's No Hard Feelings at the end, instead calling him out not only for the deception, but for asking Watson to hide in the room as a witness, and then forgetting about him.
 * Holmes does this almost constantly, to almost every single character. A fundamental part of his Insufferable Genius character.
 * Not to mention that whole thing where he pretended to be dead for three years.
 * Hercule Poirot pulled this one all the time.
 * Occurs in The Light Bearer (a historical fiction novel about ancient Rome). The male protagonist allows his aunt Arria to think her children have died in a fire in order to save them from abduction by Nero. Since Nero was in the room when the announcement was made, the protagonist knew he would not have been fooled if Arria had not expressed real shock and grief.
 * The entire plot of Mordant's Need by Stephen R. Donaldson revolved around.
 * Lucky Starr pulls this all the time, often dramatically accusing the wrong suspect on purpose and using people's reactions to gain proof against the real culprit. The worst instance is in Lucky Starr and the Rings of Saturn, in which Lucky allows everyone to believe he is going to betray Earth—including his best friend, who thinks Lucky is turning traitor in exchange for the friend's life.
 * In the last few pages of The Hunger Games: Catching Fire, it turns out that Katniss' friends kept her in the dark about some pretty huge things,  because Katniss' poor deception skills and Chronic Hero Syndrome would have ruined their plans if she had known.

Live Action Television

 * Stargate SG-1 uses this at least three times:
 * "Crossroads": SG-1 is kept in the dark by the Tok'ra.
 * "Shades of Grey": Everyone is kept in the dark by Hammond, O'Neill and a couple of alien races.
 * "Dominion": Vala is kept in the dark about her own plan (thanks to a memory-altering device) to fool her mind-reading Big Bad daughter Adria.
 * The first episode of Hustle with Danny (and the audience) only let in at the end.
 * Happens a great deal in Hustle actually, due to a combination of the Unspoken Plan Guarantee, in-character Enforced Method Acting (as in the first episode, where the realism of Danny's reactions to the unfolding situation were vital. It was also a trick to test his loyalty, as in the next point) and, in some cases, the characters tricking each other (as in the Season 3 episode where Mickey and Danny get dumped naked in Trafalgar Square for a contest to determine the leadership of the crew).
 * The Buffy episode "Enemies" uses this as well; Angel fakes turning evil to trick Faith, and really punches Xander (who was clueless) in the face during the masquerade. Whether the pleasure he took in doing so was real is left ambiguous.
 * In an episode of Babylon 5, Londo leads Vir to believe he is planning to kill G'Kar, when in actual fact it was part of a greater scheme to make rival Lord Refa (who has a telepath on his payroll) think that was the plan and go after G'Kar first, so that G'Kar could kill Refa for Londo.
 * Cheers did this a lot, such as the episode in which Coach and Harry the Hat conspired to recover Coach's money from a con by Harry pretending to betray the coach. Diane whined that they would have helped, but Harry replied, "You weren't smart enough to pull it off."

Manga and Anime

 * The Locked Room Mystery in Suzumiya Haruhi was all set up by the organization. Koizumi could have told at least Kyon what was going on, but... In the novels, it's not Haruhi who figures it out but Kyon, who then tells Koizumi that he's not as stupid as he acts.
 * Lelouch pulls this at the end of Code Geass.

Radio

 * The radio drama of Return of the Jedi contains one of these, though it wasn't explained in the film. C-3PO was deliberately kept in the dark about Luke's complicated plan to rescue Han Solo from Jabba the Hutt, so that when Jabba had Threepio's data files scanned and found no trace of the stratagem, he accepted the droid's story at face-value. (R2-D2, meanwhile, was in on the whole thing.) Leia later apologizes to Threepio for the deception.

Video Games

 * Done by The Illusive Man quite often in Mass Effect 2.

Webcomics
"Elan: Why didn't you tell me anything about it, though? Lien: Because we wanted it to work! Seriously, how many times do I have to go over the "Good, not dumb" thing?"
 * In The Order of the Stick, Elan is not told about a plot to catch Therkla in an Engineered Public Confession.

Western Animation

 * The Men in Black cartoon. J is kept out of the loop on Zed's phony retirement, so that the alien frankenstein Alpha will read J's mind and believe the lie. Unfortunately for the MIB, J had his doubts, and Alpha saw through the deception.
 * An episode of Transformers: Beast Wars had Rattrap switch sides, which was set up by Optimus and himself so they could find out how the Predacons were tapping their transmissions.
 * In Teen Titans,.
 * Similar but different application in an episode of My Little Pony Friendship Is Magic. Rainbow Dash discovers that her friends have a weekly "pet playdate" and asks why she's never found out about this. They thought about telling her, but she A) doesn't have a pet and B) tends to be napping around the time they have the playdate, so they figured she wouldn't really care one way or the other if they told her. She concedes the logic, then decides to get offended at the implication that she'll never have a pet with whom to participate.
 * Raimundo pulls this off against Hannibal Roy Bean in one episode of Xiaolin Showdown