In fiction (comedic works especially), if a character is told to "act natural" to avoid drawing attention to what is supposed to be a covert task, there's a good chance they'll overdo it. Common flubs include reading a book upside down or Not So Innocent Whistling.
May be related to the Centipede's Dilemma, where a conscious attempt to do something is inexplicably harder than doing it unconsciously.
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Anime & Manga
Dragon Ball Z has Frog Ginyu trying to steal one of the Dragon Balls. When Gohan spots him, he starts leaning against the ball and whistling nonchalantly, which only makes him more suspicious because he's a frog.
On at least two occasions in Baccano!, Isaac advises Miria that the two of them should act casual to avoid suspicion. Isaac and Miria's idea of "acting casual" inevitably makes about as much sense as anything else they do - which is to say, none.
Yu-Gi-Oh!: Since he and Anzu/Tea are stowaways on the ship to Duelist Kingdom (and Joey wasn't really supposed to be there either), Honda/Tristan advises everyone to "act casual" going past the guards...and then marches past looking ridiculously uncomfortable and suspicious. The guards notice this and call out to him... to tell him not to be so nervous.
In Hawkeye #2, Clint and Kate infiltrate a swanky gathering of underworld types, where Clint feels distinctly out of place:
Clint:(to himself) Casual. Casual. Super-casual.
Kate: You know there's nothing casual about a guy muttering "casual" to himself over and over again, right?
Film - Animated
Cars. McQueen wants to surprise Sally with his new look.
Mater: Here she comes! McQueen: Okay, places, everybody! Hurry! Act natural. [McQueen hides and everybody else gets in a perfectly straight line as Sally approaches] Mater, Ramone, Flo, Luigi, Sarge, Fillmore: Hi, Sally! Sally: All right, what's going on?
In Toy Story 2, Woody tells Buster the dog to "Act casual" while Woody hangs on his collar to rescue Wheezy unseen by humans. Buster begins to strut leisurely, and Woody hisses, "Not that casual!"
Anna:[doorbell rings] Party guests! Clark:[holding bloody shovel] Act natural. Anna and Lewis:[line up behind Clark at the door with weapons ready]
Star Wars: Return of the Jedi, when flying on approach to the moon on Endor in a stolen imperial shuttle, Han tells Chewbacca: "I don't know... fly casual."
Parodied on the Family Guy spoof, where the shuttle spontaneously starts walking with its "arms" on its "waist" and whistling.
Darth Vader/Stewie: Where is that very casual looking shuttle going?
Invoked in the second Sherlock Holmes movie. Watson is looking for a disguised assassin, eventually discovering him by knocking a tray of drinks out of a waiter's hand - the impostor is so focused on acting naturally that he can't realistically act surprised.
Animal in The Muppets when he is standing outside Jack Black's trailer to lure the actor out, as seen in the page quote.
In Johnny English: Reborn, when Tucker confronts the mole at gunpoint in front of Johnny in a washroom, an unsuspecting old man walks right in. This basically leads to the mole drying his hands, Tucker washing his hands and Johnny taking the same whiz continuously until he leaves.
The agent who follows Erik down the stairs in Hanna.
In Picture Perfect, it's actually amazing that Ernie did notice how unnatural the supposed family was acting most of the time.
An unintentional example in Birdemic - early in the movie, the actor who plays Rod couldn't walk naturally on camera and ends up looking like he's made of wood.
Lampshaded by the narrator of Small Gods: "Beneath the temple, Ern and sergeant Simony made their way through the tunnels of the citadel using the kind of nonchalant walk which would draw detail and arrow-sharp attention to them in seconds. Fortunately the guards were all above ground at the ceremony."
The same thing happens in the sixth book when she has to act casually in Borgin & Burkes and gets noticed immediately.
Live Action TV
In How I Met Your Mother, when the protagonists try to work out who Barney's stalker is, they're told to try to look inconspicuous, and Marshall immediately reacts by acting very strangely, having no idea what to do with his hands, etc.
When Rachel and Joey are having their brief fling in Friends, Ross knocks on the door to Rachel's hotel room; she tells Joey that they aren't doing anything suspicious and he just needs to act "nonchalant". He poses awkwardly with his hands on his hips, staring at the ceiling, until admitting he doesn't know what "nonchalant" means.
In Frasier, Frasier and his family are encouraged to do this by Martin after they accidentally cross the Canadian border, unaware that Daphne's work permit forbids her from leaving the country, and must pass through customs before re-entering. This eventually involves Frasier babbling his head off whenever the customs officer asks him a question, Niles buttoning up entirely, and Daphne saying the only thing she can say in a vaguely convincing American accent, which happens to be the word "Sure!" Martin, who actually does manage to act naturally, is not greatly impressed.
In Alas Smith and Jones, Mel Smith tells his friends that Queen guitarist Brian May is going to visit. They all get overexcited at the prospect of meeting him and Mel insists they treat him as a normal guy and his visit is nothing special. However, they take this overboard and when Brian arrives, they're all doing strange activities like vacuuming the flat while totally ignoring him.
In the Korean DramaBest Love, two actors who detest each other pretend to be best friends in front of the fangirls.
Buffy the Vampire Slayer - when Willow investigates the past of Buffy's newly-arrived old friend and early crush, she feels guilty about it and tries to act naturally around Buffy - failing spectacularly, but Buffy shrugs off her squeaking high-pitched panic as the result of too much coffee.
Rentaghost: Telling medieval jester turned poltergiest Timothy Claypole to act like an innocent bystander was never going to end well.
Protect The Boss has a couple of examples. Once with the Cha cousins acting friendly for the press, to cover up that they're mad at each other. And when Chairman Cha and Shin Sook Hee are doing community service together. "Oh, hello, Sister-in-law." "Yes, Brother-in-law?"
Shut Up Flower Boy Band: Ji Hyuk and Pyo Joo are fighting in gym class but when a teacher comes over, the two quickly hug.
Parodied in Supernatural: Dean and Sam are transported to an alternate universe, where they are mistaken for the actors Jensen Ackles and Jared Padalecki, who are currently filming this very episode of Supernatural. They decide to play along, failing miserably in the task of portraying their respective characters. Lampshaded by Robert Singer, who remarks that they apparently lost any shred of talent.
Subverted in The Dick Van Dyke Show. Alan Brady orders Rob and Laura to hold a party at their house for a documentary about him. Rob instructs the partygoers not to go crazy, and to treat Alan like one of their circle of friends. They do so comically: when Alan enters the front door, everyone turns, waves "hi" quickly, then turns back to their business. In unison! Alan insists on reshooting his entrance, they do it again, and Alan is furious. Not because everyone is unconvincing, but because they are NOT fawning over him. He's a big star! Why would he want to show people treating him like some ordinary shlub? They shoot it again, and this time everyone rushes to greet him.
Agents of S.H.I.E.L.D.: Simmons makes a hilariously bad attempt to act casual (coached by Skye through an earpiece) when Agent Sitwell catches attempting to access a computer terminal without authorisation in "The Hub". After first claiming to be looking for the bathroom only to be told that she is staring at a wall panel, she then attempts to compliment his head. It gets so bad that Skye has to tell her to stop talking.
In a FoxTrot strip, Paige and Nicole shoplift a CD. Nicole tells Paige to act naturally. Paige holds her breath. Nicole tells Paige that acting naturally includes breathing. Paige immediately starts loudly panting as Nicole Face Palms and adds "Quietly".
In the play Ramona, a maid is eavesdropping at a door. She's holding her broom bristle-end up. When someone comes up behind her and says "What are you doing?", she says "Sweeping!", and makes sweeping motions with the upside-down broom.
Lisa Simpson: It's the mother ship! Bart Simpson: This must be the level boss for this part of the game. Homer Simpson: The boss! Quick, act natural. [puts on business glasses and pretends to write on a clipboard]
One sequence in Sakura Wars: So Long, My Love, where you're tracking a mafiosi through New York along with one of the girls. At one point, the mafiosi's bodyguard gets suspicious and hangs back to check on you. You can choose to 'Act Natural' and deflect attention... or you can deliberately act outrageously suspicious, which works even better, and makes the girl impressed by your Genre Savvy.
In The Sims Medieval, a Spy who's doing something like eavesdropping or stealing from a messenger post will do a "stealth walk" that's so over-the-top it should logically be more noticeable than a natural walk.
An early level in a Hamburg nightclub in No One Lives Forever has Cate being followed by a HARM spy. The tail is immediately suspicious with his fedora and raincoat, and Cate mentions that he's so obsessed with following her blindly that he'd probably wander into traffic just to keep up with her. As expected, he follows her right into the women's restroom and gets himself iced.
In one strip of Public Humiliation, the children do exactly this — and the adults who see this see right through it, only to make the wrong conclusion — that one of the children is hoping the adult will not remember the overdue library book.
In Red vs. Blue, Grif and Simmons have to pretend they haven't seen Epsilon anywhere, nope, definitely he wasn't here just a minute ago until we punted him over the hill over there... Grif does fine. Simmons, not so much.
Simmons: Whoa! I mean, what's the four-one-one, daddy-o's? Me and my homey were just, hangin' all up in here. Deserts-ville. 'S'all good. You know what I'm sayin', scrillas?
Grif: What the fuck are you doing?
Simmons: You told me to be cool.
Grif: Right. So what the fuck are you doing?
Happens briefly in BAMF Girls Club when Hermione tries to sneak 50 Shades of Grindelwald off a shelf.
Exaggerated in the episode "Life of Crime." Spongebob and Patrick are panicking because (they think) they stole a balloon. Spongebob advises, "Just act natural," and, whether out of nerves or sheer stupidity, both of them start gibbering and acting crazy. They are immediately surrounded by a crowd who mistake them for street performers.
In the episode "Boating Class", when Spongebob is asked to act natural, he gets on all fours and starts grazing like a cow.
In another, he becomes a live-action sponge floating in the toilet.
In Family Guy Presents: Laugh It Up, Fuzzball, Peter-as-Han cautions Chris-as-Luke to "play it cool" when they're sneaking off to rescue Lois-as-Leia. This apparently entails slouching along jazzily to the strains of Cab Calloway's "Minnie the Moocher".
In reference to the above Star Wars quote, when Han tells Chewy to "fly casual," the shuttle proceeds to strut past the Star Destroyer.
Stewie-as-Vader: "Where is that casual looking shuttle headed?"
In Avatar: The Last Airbender , in the episode where they met Haru, the Gaang have followed him, and when a group of Fire Nation soldiers barge in they all "act natural." This involves freezing in place.
Skinner and Chalmers do it in The Simpsons episode "500 Keys" while waiting to snatch the key to the hidden classroom off Lisa.
Also, Homer and Milhouse do this in 24 Minutes:
Homer: "Cops! Act natural."
Homer: (Imitating a woman) "Oh hello officer!"
Chief Wiggum: (While driving by) "Oh hello, Ma'am!" (To self) "What is it about a women and a dumpster?"
Sym-Bionic Titan: In "Lessons in Love", Newton and Kimmy are about to kiss when Lance and Ilana burst in. Newton randomly shouts out the word "Parallelogram!" and extends his book pages forward towards Lance and Ilana, while Kimmy opens her book upside down.
Taken to ridiculous extremes in the Earthworm Jim episode "Upholstered Peril", where a scientist observes the fibers in an evil sofa that brainwashes people into becoming couch potatoes. Said fibers turn out to be alive, and one of them shouts "We're being watched! Act natural!", followed by the fibers indulging in Not So Innocent Whistling.
Pinky and the Brain: In the Christmas Episode, the duo hide themselves in Santa's reindeer team, wearing antlers on their heads. The Brain tells Pinky to just act natural. Cue Pinky moving erratically, singing, laughing, and making his usual Verbal Tics.
The Brain: Pinky! Not that natural!
Finn indulges in a bit of this in the Adventure Time episode "Storytelling".
"Normal guy, forest guy, just like normal forest guy..."
His efforts at playing it cool in "Slumber Party Panic" are also very, very awkward and unconvincing.