Paper People

""Being paper thin, I'll just slip in quietly through the crack in the door! Hahahahahaha!""

- The Infamous Paper Doll Man

This is when a character is quite literally paper-thin, as if they were a paper cutout. This can also apply to normally three-dimensional characters who have been flattened as a result of, say, a 300-pound weight being dropped on them.

Not to be confused with Flat Character.

Comic Books

 * Calvin once imagined he had become two-dimensional, allowing him to escape notice by turning sideways.
 * At the price of only being able to move by either "flopping" his body or just going along with a breeze.
 * Sam and Max undergo this at one point, in order to get under a locked door to obtain a Super Mario Bros.-styled coin.
 * Max: Lose weight and make money? Where do I sign up?
 * Sam: I dropped a whole dimension and I've never felt better!
 * Flat Man of the Great Lakes Avengers
 * One robber / rapist crushed by a millstone in a story by Wilhelm Busch. Other than typical for this trope, he doesn't exactly revert.

Film

 * In the new version of Charlie and the Chocolate Factory Mike Teavee and then has to be  leaving him very tall but no where near average girth.
 * Judge Doom towards the end of Who Framed Roger Rabbit?.
 * Reedman from Transformers: Revenge of the Fallen.
 * A Nightmare on Elm Street:The Dream Child has Freddy turning Mark into a paper cutout.

Literature

 * Flat Stanley is about a boy who is flattened by a falling bulletin board.
 * Flatland has an entire paper-thin WORLD.
 * Even more so, A.K. Dewdney's Planiverse.
 * In one scene in A Wrinkle in Time, Meg finds herself transported briefly into a two-dimensional world, where she can't breathe ("a paper doll cannot gasp"), and her heart can't pump blood properly ("a knife-like, sideways beat"). It was, Mrs. Which explains, an oversight on her part (apparently she and her two companions were perfectly comfortable there).
 * In Pyramids, one of Ptaclusp's sons accidentally becomes this trope due to the twisting of dimensions by the grossly-oversized Great Pyramid. He also tends to drift horizontally at a steady rate, as the "fourth dimension" of Time now runs that way for him.
 * The Beautiful Culpeppers is a children's book about a family of paper dolls owned by a little girl. They also have a 2-D paper house, which is tacked to a wall; they can go inside it, but we never get any details about what it's like in there.

Tabletop Games

 * Grimm has the flat folk—people who were crushed under the falling Beanstalk but weren't killed by it, and were squashed into two-dimensionality. Well, physical two-dimensionality. Humans native to the Grimm Lands are always two-dimensional in the literary sense.

Video Games

 * The universe of Parappa the Rapper is inhabited entirely by paper people.
 * In Paper Mario, almost everyone is literally a paper cutout.
 * Turned into a gameplay element in the sequel, Paper Mario: The Thousand-Year Door: Mario can turn into a paper airplane, turn sideways to fit through cracks (and even has to hop to move around in said form) and several other curses.
 * Mr. Game & Watch
 * Everyone in Harvest Moon: My Little Shop.
 * The Flatso enemies in Banjo-Tooie, which pop out of the floor of Cloudcuckooland's Central Cavern.
 * the fact that they make a 'cha-ching' sound like that of paper money adds to this effect.
 * Hype the Time Quest has tapestries with pictures of soldiers on them in the monastery. When you walk by them or retrieve something or otherwise trigger them, the soldiers rip themselves off the tapestries and attack you, still flat as paper. Fortunately, them being paper/cloth, fire magic tends to work well against them.
 * The Darwinians of Darwinia look like paper cut-outs. The basic Virii are just moving patterns on the ground.
 * Cheat codes will let you do this to Spyro the Dragon in the PS 1 games Ripto's Rage and Year of the Dragon.

Web Original

 * This Whole Movie is built off this trope.
 * "The Reddish Radish" animation from Homestar Runner.
 * Mister Origami, a World-War II-era supervillain from the Global Guardians PBEM Universe, was paper-thin and could fold himself into many different paper animal shapes... and assumed the abilities of the animal in question.
 * The Paper Pleasers of the Ever After in Volume 9 of RWBY are literal walking, talking origami constructs.

Western Animation
"Horace: It's like we're cartoons! How horrible!"
 * Pettanko Kate Moss was portrayed this way on Family Guy.
 * In Foster's Home for Imaginary Friends, the character Duchess appears like this whenever she turns around.
 * Paper Doll Man of The Impossibles, who used his powers to steal top secret plans from the Pentagon, as well as money.
 * The protagonist of Thin Pig makes better use of this than most, folding himself like origami into whatever form is necessary at the moment.
 * From a show within a show on Futurama: "Father! The kidnappers cut off one of my dimensions!"
 * Also, during The Beast With A Billion Backs, Fry and his date went on a 2D Tunnel of Love ride.
 * On Ugly Americans, a wizard rival of Leonard's is trapped between dimensions, manifesting himself as a paper cutout.
 * In the Classic Disney Short "Pluto's Judgement Day", one of the cats testifying against Pluto is a chubby kitten whom Pluto chased into the path of a steamroller. After testifying he turns around, revealing that he - and the balloon he's carrying - is flat as a pancake.
 * During the first Jimmy Neutron/Fairly Oddparents crossover special, when CG Jimmy is transported into Timmy's 2D world, he remarks that his depth is gone, and falls flat on the floor like a standee.
 * Teri the bear, a character from The Amazing World of Gumball, looks like a crumpled paper cutout.
 * This Kaput and Zosky episode heavily features this.
 * In The Problem Solverz episode "Zoo Cops", the characters are transported to another dimension, where everyone becomes this.

Real Life

 * Flatworms
 * Ediacaran biota
 * Trichoplax adhaerens, sole member of the phylum Placozoa
 * Plane trees