Accent Relapse

"I see you supized I speak Engrish so werr!"

- Dave Barry

When a character who has been speaking perfect, unaccented English (for whatever region he's in), gets revealed to be a spy or covert operative from some other country, from that point on, he instantly reverts to a thick accent from his native language. A character revealed as a robot will suddenly switch to a classic "robot" speech pattern.

This is distinct from Ooh, Me Accent's Slipping in that it is not a poor acting display by the performer, but instead a deliberate (and inexplicable, unless the character is aware his cover is blown) thing written into the story. Bear in mind that in works using a Translation Convention, the transition from fluent English to, say, Russian-accented English may represent a genuine transition from English to Russian.

From a linguistic standpoint, it can actually make sense, as maintaining an accent or dialect perfectly is exceptionally difficult until you get extremely familiar with it. Most people would cease to exert the effort if they knew they didn't have to.

Film

 * Spies Like Us: Once the are revealed to be Russian spies, in the subsequent interrogation scene, they have thick Russian accents.
 * The Rocketeer: The German spy,, after speaking perfect English the whole movie, ZUDDENLY HAZ ZE GERMAN ACCZENT VUNCE HE IZ EXPOZED!
 * Norm in There's Something About Mary: he even forgets himself and nearly uses his real accent in front of the woman he was trying to fool, before trying to recover but giving up and telling the truth. Justified, in that he was putting on an upper class English accent and is actually American.
 * Done by Ann-Margaret in Neil Simon's The Cheap Detective, when detective Peter Falk tricks her into revealing that she's Russian -- she is unable to pronounce the word "baubles".
 * Inglourious Basterds: In a case of Language Relapse, reverts to speaking  after he is discovered. Slightly muddied because his imperfect accent when speaking  was part of what raised suspicions against him in the first place.
 * What actually gave him away was when placing an order for three scotches, he physically gestured with his index, middle, and ring finger, rather than his thumb, index, and middle.
 * Another language relapse is done in The Great Escape to tragic effect:
 * Not a spy, but in Watchmen Adrian Veidt/Ozymandias switches from a perfect American accent to a German one after . In this case it's not that his cover was blown so much as he was so emotional he stopped caring.
 * He actually speaks with the accent earlier when he's with his fellow "Watchmen". He uses an American accent only when speaking to employees, business associates and reporters.
 * Was supposed to happen with Dr. Scott in The Rocky Horror Picture Show, but for some mystifying (to O'brien and the audience alike) reason Jonathan Adams had an accent even before being called von Scott.
 * Done in Spy Hard, after Dick Steele (agent WD-40) makes love to a woman, she is revealed to be a spy for General Rancor with a heavy Russian accent. Apparently, she doesn't slip into her native accent even in the throes of passion but drops it immediately before trying to shoot Steele. He doesn't notice it, though.
 * Inverted in Psycho Beach Party when

Literature

 * This is inverted in The Mote in God's Eye. One character (the Chief Engineer, no less) usually speaks with an exaggerated Scottish accent, and like all inhabitants of 'New Caledonia' places great emphasis on his proud Scottish heritage, despite this being the far future when there isn't much left of the old cultural distinctions. Another character remarks that he loses the accent when he gets excited, which he loudly declares a lie--without using the accent.
 * In The Lies of Locke Lamora, Locke drops his false Vadran accent the few times he's completely found out.
 * In rare moments of overwhelming emotion--usually fear or rage or both--Felix in Doctrine of Labyrinths will lose his posh accent and slip into a Lower City one. Ultimately he will always begin using his fake accent again the moment he notices.

Live Action TV

 * Poirot did this once with a supposed socialite who reverted from RP to a Cockney accent after being exposed.
 * Possibly a Shout-Out, the same thing happens in "The Unicorn and the Wasp", the Agatha Christie homage episode of Doctor Who.
 * When the King of the Renaissance Faire is deposed and hit with a sexual harassment lawsuit on King of the Hill, he immediately reverts from Ye Olde Butcherede Englishe to a thick Texas accent.
 * Even funnier when you realize the voice actor in question was Alan Rickman.
 * On CSI: NY, a man pretending to be Jewish is revealed to be a German former Hitler Youth soldier. Even after 64 years of pretending to be a non-practicing Polish Jew, marrying a Jewish woman, and raising an Orthodox Jewish son, he reverts to his German accent when his crime is revealed.
 * Happens to a German spy who had tried to infiltrate Hogan's Heroes.
 * Spoofed in Get Smart. Max asks why The Mole is suddenly speaking in a fake German accent after being unmasked. He replies, "Vot accent? Zis is my veal voice!"
 * Inverted on an episode of Bones: the intern with the thick Middle Eastern accent slips into his actual American accent. It turns out that he was faking the accent to avoid having to explain to the practically Straw Atheist Brennan and Squints why he took his religion so seriously: if he was an immigrant, it would make sense for him to clutch at his roots, but he apparently felt that the members of Brennan's team wouldn't be able to understand why a rational, scientific mind like theirs could not only be religious, but devoutly so.
 * More of a Personality Relapse, but before being revealed to the audience as a Terminator in The Sarah Connor Chronicles, Cameron talks and acts like a perfectly natural teenage girl. Afterwards, even in situations when she's with people who don't know she's a Terminator, she seemingly loses this ability to act natural and instead stares at people oddly and is generally robot-y in her speech patterns and reactions.
 * Averted in Blackadder Goes Forth, where a British spy who has been spying on Germany returns and retains his extremely heavy German accent, causing others to assume he was a German spy.
 * Averted in the classic Star Trek the Original Series episode "The Trouble With Tribbles" -- when "Arne Darvin" is found out to be a Klingon spy, his demeanor remains pretty much the same humanish demeanor that it was. (When he returns for Deep Space Nine, he does go just a bit hyper when he's transferred from Cardassian custody.)
 * Even that is explained away by finally getting a caffeine fix (the closest Cardassians have to coffee is a fish juice).
 * In season 8 of Twenty Four, the assassin Davros, in his cover identity of an NYPD officer, speaks in a New York accent when trying to get a coworker to switch shifts with him, but once it becomes clear that the guy won't budge, Davros pulls out a gun and switches to a Russian accent.
 * In an episode of Chuck, the titular character encounters a British handler (played by Timothy Dalton) who turns out to be the criminal mastermind Alexei Volkoff. When he reveals himself, he suddenly switches to a very bad Russian accent. In the subsequent episodes, though, he speaks with a perfect British accent, even in his Moscow headquarters. Later explained when he turns out to have been British all along, just brainwashed to think he was Russian.

Other

 * In the Clue VCR game from the 80s, Miss Peach reveals herself to be M. Brunette's daughter and therefore French. She subsequently delivers most of her remaining dialogue with a French accent (as opposed to the Southern drawl she'd used until then).

Video Games

 * In Freedom Fighters, once is revealed to be a Russian general, he is calling out to you through a sewer. Your character does not recognize his voice until he uses his disguise American voice once more.
 * In BioShock
 * Ace Attorney: Though not a spy, Maximilian Galactica speaks in a very Camp Straight manner, until things start to get a little to stressful. Then he breaks down and starts weeping and talking with a heavy southern accent.
 * Presumably this happens with culturally-appropriate accents in Japan, as in both versions Max is a country bumpkin who made good.
 * When The Spy in Team Fortress 2 is disguised, his voice clips change to match that of whoever he's disguised as. Which is more a gameplay mechanic, but fits the trope because he reverts to his normal accent once he drops the disguise.
 * Final Fantasy XIII: When Vanille first shows up, she slips in and out of an Australian accent to the point where it sounds like it's a bad case of Ooh, Me Accent's Slipping, but after, she gets the accent full-time, matching Fang's.
 * In Star Wars: The Old Republic, the Imperial Agent normally speaks with a thick british accent common to the rest of the empire, but when he goes undercover he drops the accent completely. If his cover is blown at any part of the story he drops the accent once he's positive they know he's a spy and not just bluffing him.