A smile is supposed to be a reassuring expression, indicating happiness and complete absence of angst and danger. If someone's smiling, it means all is well with the world...
...unless it's a Cheshire Cat Grin, which always denotes that someone's in for it.
The Cheshire Cat Grin is an expression made by a Trickster who's up to something, and "something" never bodes well for the person he or she is smiling at. Usually, it involves their total humiliation — occasionally it involves mortal danger. A typical scenario runs something like this:
Character A: Wait, we need someone to dress in a bikini, swim across a river full of piranhas, placate the pride of man-eating lions with the power of dance and steal the sacred ruby out from under Lord Evil's nose? Who's the poor fool who's gotta do all that? Character B:(grins at character A) Gilligan Cut to character A dressed in a bikini swimming across a river full of piranhas to placate the pride of man-eating lions with the power of dance so he can steal the sacred ruby out from under Lord Evil's nose.
Generally, this is drawn as a smile with far too many teeth on display, where the mouth doesn't match up with the eyes, going practically from ear to ear. For example, the mouth may be a broad grin, but the eyes definitely have a wicked gleam in them. It occasionally has sexual connotations, particularly when there's a predatory quality about the grin.
Strangely, it's usually done by heroes who have a Zany Scheme in mind, rather than by villains, who tend to prefer the Psychotic Smirk or the Slasher Smile. Heroes and heroines who are slightly eccentric — or slightly deranged — favour the Cheshire Cat Grin. If it's an anime, expect a Cat Smile instead. If this smile is a permanent feature, it's a subtrope of Frozen Face; if it's cut into the character's face, it's a Glasgow Grin. Shares living space with The Un Smile.
If you ever see someone with the Cheshire Cat Grin in real life, vacate the premises before you can discover what they're smiling about. Or, for that matter, how their face manages to do that.
Gengar, from Pokémon, tends to have either this or a Slasher Smile depending on how it's drawn. Either way, it is constantly worn and only twice in the anime Gengar actually frowned.
See the example under Video Games for a time when he does frown.
Dee Laytner, from the FAKE manga and anime, is a good example of a grin with sexual overtones, while he's plotting how to seduce Ryo. Occasionally, Dee's expression is downright scary, and it leads Ryo to comment, "You're grinning like the Cheshire Cat." Here, have a look.
Washu of Tenchi Muyo!!, usually right before she's about to stuff Tenchi into some machine or another.
Ichihara Yuko from Xxx Ho Lic has a grin that can roughly be summed up in words as: "Not only do I know something you don't know, but it's going to bite you in the ass in a veryamusing fashion." Despite this, it's when she appears serious that should really scare Watanuki. This is because Yuko's Cheshire cat grin simply means she's going to make him do all the laundry, whereas the latter expression is usually a forecast of man-eating crow demons or similar.
Deviant genius Hiruma from Eyeshield21 has this as one of his default expressions, to the point where those new to the series — or complete non-fans — can identify him as 'the one with the teeth.'
Road Kamelot of D.Gray-man, several times. Along with some of the AKUMA. Hell, the Earl probably counts as well.
Haruko of FLCL, usually when she's about to be entertained, between the Cat Smile and Slasher Smile stages. It's very, very much a bad thing for Haruko to be entertained.
Mahou Sensei Negima!! plays this for comedy; if Haruna starts smiling, something embarrassing and/or unpleasant is about to happen. The other characters are rightfully scared of her.
Higurashi No Naku Koro Ni has a few characters sport these. Most of the time it's either Mion or Satoko, and it usually involves getting Keiichi into a ridiculous outfit, or out of his clothes entirely.
Not to mention Ciel. When he fakes a smile, it's adorable. When he gives a real smile... It probably means someone is about to get hurt. Badly.
And in one of the OVAs? Grelle literally had the Cheshire's smile while playing the Cheshire Cat!
Bel from Katekyo Hitman Reborn! tends to have this as his default face, along with making one when he laughs his creepy "Ushishishishi~" laugh.
Hmmmm... notice how his default outfit is actually a pink and purple striped shirt? And how in one Omake he's considered a house pet in the Varia family? Looks like the resemblance isn't just the smile...
Naruto Uzumaki smiles like this a few times, usually when thinking of a prank. Some of those times (notably in the 101 episode) his Cheshire Cat Grin overlaps with Slasher Smile.
The Kyuubi's face is eternally twisted into a Cheshire Cat Grin.
Ham Egg from Osamu Tezuka's works is easily indentified for making this grin constantly, which foreshadows his unpleasant and villainous nature.
Fullmetal Alchemist: Truth usually has a grin that's wider than seems possible when seen and its the only feature other than his body's outline that's readily discernible. The lack of eyes makes it hard to decide what kind of smile it is, though.
The Gold-Tooth Alchemist echoed this, often wearing a bizarrely-wide grin at times when there was no discernible reason for anyone to be smiling. Add the unfocused eyes and it moves into Slasher Smile territory.
Neon Genesis Evangelion: Asuka pulled one off in her debut episode when Gaghiel attacked the UN fleet carrying her Eva.
Shinji: This is really bad! We've got to go back and find Misato! Asuka:(looks sideways and grins) Wunderbär!
Kaworu often wears one himself, particularly in the manga or any of the other alternate universes where he pops up. It fits in well with his cryptic nature.
Totoro himself smiles like this when he is happy or delighted by something.
Films — Live-Action
The Machinist: Ivan does a particularly disturbing rendition of this trope.
Pirates of the Caribbean: Right before taking vicious or manipulative action, Captain Jack Sparrow often sports an evil, toothy smile. In the screenplay for Dead Man's Chest, a "Cheshire grin" is mentioned in Jack's direction during a particularly loathsome act of betrayal.
The allegedly-catatonic Cameron, just pulled up from the bottom of a pool after keeling over into it face first, broke into one of these in Ferris Bueller's Day Off when Ferris and Sloane realize he saw Sloane stark naked while she was changing into her swimsuit, and enjoyed every minute of it. Cameron's ensuing hysterical laughter led to further dunkings by Ferris.
Dr. Frank-N-Furter walks on stage with this expression in the Rocky Horror Picture Show, and it's his default expression when he's not angry, jealous, megalomanical, or begging for sympathy. No real surprise really, as Cheshire Cat is actually a nickname for the actor Tim Curry.
Similarly in his role in Home Alone 2: Lost In New York. They even segue quite unnervingly from his grin to a still from the animated version of the Grinch — the resemblance is uncanny.
Equilibrium's Cleric Brandt has a smile which first inspires Fridge Logic when you realise that he's not supposed to have any emotion, then Fridge Brilliance when you realise that he doesn't have any emotion; his huge smile has nothing to do with real happiness or real satisfaction. Knowing this makes it pretty creepy.
Zero Mostel as Pseudolus delivers one of these to Jack Gilford's Hysterium in A Funny Thing Happened on the Way to the Forum. They've determined that the only way to get a visiting Captain to leave is to convince him that his intended bride is deceased. Hysterium intones, "But where are we going to get a body?" whereupon Pseudolus does a slow take at him with a grin that almost breaks his own face.
Cap:*having just told everyone else what to do, in detail, looks to Hulk* Hulk? Hulk:*looks down at him* Cap:*points up at the Chitauri invaders* Smash. Hulk: >:D
The movie version of How the Grinch Stole Christmas also has the scene where the Grinch grins at his "wonderful, awful idea", except also the Slasher Smile one is done in an extremely nasty way during the "You're A Mean One Mr Grinch" musical number, the "You have termites in your smile" line and we are treated to a closeup of his mouth and we see his teeth which have bugs crawling around in them.
The Trope Namer is the Cheshire Cat◊, from Alice's Adventures in Wonderland. He's a mischevious creature, but not malicious to Alice or anyone who doesn't seem to deserve it. He even manages to mock the Queen of Hearts and then fade away before she can retaliate. The character is a play on the phrase "grinning like a Cheshire cat," which simply means smiling broadly. No one is exactly sure where the phrase came from.
Angua in Feet of Clay gives a rioter "a friendly smile. That was to say, her mouth was turned up at the corners and all her teeth were visible."
From Wyrd Sisters: "Greebo's grin gradually faded, until there was nothing left but the cat. This was nearly as spooky as the other way round." Keeping in mind that this is a housecat who rapes wolves and terrifies bears.
"It was a smile like the one you find on black and orange striped creatures prowling through the jungle looking for someone to smile at."
Not quite a Cheshire Cat Grin, but with the same effect in "Bride of Chaotica!" an episode of Star Trek: Voyager. The holodeck malfunctions, and someone must go into the program to resolve the issue playing the part of Queen Arachnia. When Captain Janeway asks Tom "Who do you have in mind?" Tom, and the rest of the crew turn to her with evil smirks.
Talk show host/Comedian Jay Leno always seems to have one of these on his face.
Theo Kojak pulls out one of these at least every couple of episodes.
Television Without Pityrecap: I think part of the actual — not the endlessly explicit and told-not-shown, but the actual, the perceivable — charm of River is her lovely smile when she's doing something really awful.
Strangely enough, the gentle, unassuming, adorkable Fifth Doctor often wore one of these, especially when talking to people taking him prisoner. He smiled most of the time, in fact, but it very rarely reached his eyes.
In the Eighth Doctor Adventures novels, Fitz has been known to smile "with a wicked twist to his mouth that showed far too many teeth", although he's really a very Nice Guy under the somewhat sleazy exterior and it doesn't tend to mean he's really up to anything all that bad.
Vala in Stargate SG-1 has a grin that shows a lot of teeth and can be deeply disconcerting.
Michael Westen from Burn Notice does this a lot. His smile will eat your soul.
Nikola Tesla (as played by Jonathon Young) on Sanctuary.
Kamen Rider Fourze has stoic "I don't wanna be involved with this Kamen Rider Club" foolishness Kengo Utahoshi have one of these as he drags the poor Butt Monkey JK into piloting a mecha known as the Power Dizer.
Justified has Detroit mob lieutenant Robert Quarles, who has a big, unsettling smile.
Babylon 5: Mr. Morden will give you a big jovial-looking smile, just as he's about to screw you and your entire species over.
Sister Julienne of Call The Midwife, nun-in-charge of Nonnatus House, has one, which she usually deploys right after she has "asked" someone to do something. The "or else" is implied.
Smallville: Tess Mercer's smile is always a sign she's plotting something. Somewhat subverted, in that she does ave a genuine smile she can use when happy, and her intentions are always for the greater good, but when plotting, her smile can be downright scary.
In thisGarfield comic strip, Garfield tormented Jon by Cheshire cat grinning in the dark.
Another strip had him entering a room where Jon was with a huge, toothy smile and shifty eyes for no apparent reason. Jon, annoyed, asked: "All right, Garfield - what have you done?" Garfield of course remained silent, prompting Jon to keep demanding what he had done until he finally snaps and starts screaming: "WHAT HAVE YOU DONE?! WHAT HAVE YOU DONE?!" (Garfield hasn't done anything.)
Garfield: I love mental games.
One Beetle Bailey Sunday strip had Beetle torment Sarge by periodically giving him one of these smiles. Sarge tore his living quarters and office apart trying to find whatever Practical Joke Beetle had set up (in fact, there wasn't one).
In one Calvin And Hobbes arc, when Calvin notices Susie left her dolls outside while she had lunch, Calvin makes one and says "That gives me a brilliant idea!"
We also have Sableye◊ from Gen III, but it's generally hard to smile pleasantly when you have a mouthful of sharp teeth.
Tohsaka in Fate/stay night is described as being scariest when she's smiling, because then you know she is going to get you. Nobody else is ever happy when she smiles.
That's nothing. Sakura's can silence a room and extinguish candles. It makes sense. They are sisters, after all.
Mario And Luigi: Fawful's face seems to be permanently frozen into this expression. Whether he's dropping a giant rock on our heroes, fighting them off in a mech, selling them badges, or mutated by the power of a Sealed Evil in a Can into a freaky bug thing, he always has a giant toothy grin on his face.
Super Paper Mario: Good ol' Dimentio's mask/face is always curled up into an unnerving smirk. Although, to be fair, he was never even trying to be on the heroes' side to begin with, so no mystery there. Beyond that, it was more so when he ended up possessing the final boss, though that arguably made his joyful look even more unsettling.
Taokaka of BlazBlue typically makes this face whenever she wants to "play" or she's about to attack some poor fool. This being a game that centers on, y'know, fighting, she keeps the expression some 90% of the time.
Travis Touchdown of No More Heroes, particularly whenever he enters a fight while in a good mood. The loading screen for the second game even shows a small picture of him sporting one.
The Judge in OFF sports a permanent one. Ironically, he's probably the nicest character in the entire game.
Whenever The Cheat from Homestar Runner smiles, it's this kind of smile. Things often backfire on him, though.
Both Zimmy and Coyote in Gunnerkrigg Court. Oddly, the effect of each character's grin is heightened by the fact that they have the wrong sort of teeth: Zimmy has shark-like teeth and Coyote has uniform, flat teeth.
Of course, both of them pale before the awesomeness of Jack's smile.
In Girl Genius, "wild" Jagers tend to wear this most of the time: there's so much funny things to happen — like plowing through a crowd of monsters — and they have pointed teeth. Gil grins like that when he is stuffed to the gills with combat stimulants and about to blow up something.
Sparks tend to wear a huge grin when they've got an idea. Considering the stuff they usually invent, this is seldom good news for anyone in town.
Dagre from The Meek sports one...kind of. It's more of his jowls drooping, giving an absolutely terrifying grin appearance.
Subverted/defied with Florence Ambrose in Freefall — her natural smile is a face full of teeth, since she's an anthropomorphic "Bowman's Wolf." She has to remember to smile with her lips closed to avoid this trope — though she's not above invoking it...
In the penultimate episode of Teen Titans, a character named Cheshire appears, wearing a large mask with a permanent Cheshire Cat Grin on it. One of her powers was to turn invisible except for the eyes and the grin, much like the cat in Alice in Wonderland.
She's in the comics, too, minus that power. For the record, her name is Jade Nguyen, and she has done things justifying that grin (like nuking Qurac).
The Amazing Mumbo gives off a rather literal example in "Bunny Ravennote (Or How To Make A Titanimal Disappear)" .
Mumbo: Tonight, I will make the Titans disappear... (Dramatic Pause) ...forever.
In Young Justice, Jade has the mask (which isn't in the comics. The previous animated Cheshire made just that much of an impression in her less than a minute of screentime.) but not the invisibility. She's an assassin for the League of Shadows initially, but is far from Ax Crazy enough to nuke a small country out of existence For the Evulz. She also has more than one mask (since one of them currently resides in Kid Flash's trophy case.)
On The Grim Adventures Of Billy And Mandy, shapeshifter Nergal Junor replaces the teacher in order to get revenge on the class bully Sperg. "Ms. Butterbean" approaches the bully with a smile that starts as a Psychotic Smirk and quicky grows in width, as well as tooth size and sharpness, until it's grinning fanged maw as wide her head.
Timmy gets one of these himself, in the episode F.L.A.R.G. Mark sums it up pretty well.
Mark: What's with the face? It is happy, yet at the same time, disturbing!
In the Code Lyoko episode "Killer Music", Odd has one of these planted on his face for all but a few minutes of the episode... while comatose and dying.
In the Exo Squad episode "A Traitor Among Us", J.T. gets one of these when he explains his plan for getting The Squad to Venus. "We need to become official undesirables. That's one assignment you troopers might enjoy."
Henry and June (the latter more often) do this in Ka Blam!
Twilight Sparkle of My Little Pony: Friendship Is Magic pulls one off in "Lesson Zero" when she prepares to create an argument amongst the Cutie Mark Crusaders in a bid to solve the said problem and have a friendship report to write to Princess Celestia.
T-Bone from SWAT Kats donned this grin occasionally, almost uniquely amongst the characters of that series.
More often than not, Mr Bogus will very often grin like this whether the situation calls for it or not.
Chimpanzees "smile" by baring their teeth as a way of warning potential challengers that this chimp is not to be messed with. This has led to more than a few people getting mauled, because, hey, the monkey is smiling at 'em! And ain't he just like a little person? Maybe he wants a hug! NO. When a chimp smiles at you, he's telling you to fuck off. By the way, those teeth? They go for the face. And chimps are strong. Mister Peanutpants does not want to be your friend, kids.
In fact, this is true of many animals, since their teeth are their main weapons. Its meaning is lost for humans because of our greater variety of expressions and because our teeth are puny compared to theirs.
Former Australian Treasurer Peter Costello was famous for his Parliamentary savaging of his opponents in debate, of which his Cheshire Cat grin would be a prime indicator to get out of the way of his victims.
Tony Blair was known as the Cheshire cat (then the less favourable "Cheshire twat") since he became leader of the opposition and then prime minister due to his smile.