Lie to the Beholder

A spell or other mysterious effect is placed on a character to make them look and sound like a completely different person, to select people. The user is usually unaffected, meaning that the character hears and sees his or her true form at the same time as he or she is deceiving others.

A variant of Weirdness Censor. A Form You Are Comfortable With and You Cannot Grasp the True Form are often forms of this. The Master of Illusion is likely to perform this.

See also Appearance Is in the Eye of the Beholder

Anime and Manga

 * In Wolf's Rain the wolves can appear human, but they often appear human to each other even when there are no humans around.
 * This is played with in To Aru Majutsu no Index where a spell is cast worldwide that makes everyone look and sound different, but only four people are immune (and notice the difference) This is really confusing for the audience, as  and this is also the first time we meet his parents, leaving us to be unsure whether it's a dream sequence or a massive hoax played on Touma by the rest of the characters.
 * In Code Geass, where Lelouch geasses  into seeing and hearing Lelouch as.

Comic Books

 * In the Marvel Universe, the Planet Eater Galactus appears to any observer to be gigantic member of the observer's own race. What his true appearance is like is unknown.

Fan Fiction

 * Variant used in The Girl Who Lived sextet, where the eponymous Mary Sue owns a talisman that gives the appearance of clothes so that she can walk around naked with only certain people seeing her that way.

Film

 * The film Here Comes Mister Jordan (and its remake Heaven Can Wait and the 2001 remake of the remake Down to Earth) is about a man who dies before his time and is sent back down to earth in the body of a recently deceased millionaire. To the audience he looks like himself, but to the characters he looks like the aforementioned millionaire.

Literature

 * Done in The Dresden Files by Thomas to allow Harry to avoid The Fair Folk for awhile.
 * In the Backup novella, the villain hits Thomas with one of these, changing his appearance to match that of the man she'd told Harry had kidnapped her child.
 * In The Black Unicorn, the second book in the Magic Kingdom of Landover series by Terry Brooks, the hero, Ben, has a spell cast on him by an evil wizard that causes him to look and sound different to everyone except his enemies.
 * In Wizard's First Rule, Richard is at one point subject to a spell that makes everyone see him as an enemy. To his enemies and himself, he looks and sounds normal, but to his friends, he appears as someone they hate.
 * The disguise spells in the Myth Adventures series appear to everyone but the caster.
 * Which can be inconvenient: you don't know whether you're not seeing the disguise because you cast it, or because it failed. So far, I don't think it's become a major plot point, but it did happen at least once.
 * It did become a major plot point in one novel but was kinda overshadowed by the fact that the main character was insanely low on magical power.
 * And don't forget that at one point, Skeeve completely forgot that he was still disguised as a Pervect when he started to talk about who he was...
 * In Broken Sky, Gerdi's spirit stones allow him to disguise himself as anyone, but can only work on so many people at once. Thus, he'll often be walking around looking to the townsfolks one way, while still looking like his true appearance to those who know him.
 * The Talismans of Shannara features the Mirrorshroud, a cloak which makes the wearer appear to be someone the beholder trusts when worn
 * In Fred Saberhagen's Books of Swords, this is the main power of the sword Sightblinder. Its wielder appears as someone the viewer either loves or fears. The downside is that this makes it very hard for the wielder to be inconspicuous.
 * In at least two instances in the Star Wars Expanded Universe (in Solo Command and Heir to the Empire) protocol droids are modified to sound like other people, either to distract the enemy with a decoy or cover the escape of the real person (or both). Obviously they still look like droids, although Squeaky in Solo Command is outfitted with a mask and uniform to fool casual observers.
 * In the Young Wizards series, when Dairine is taking a trio of extraterrestrial exchange students on a tour of the local mall, the walking Christmas tree and the giant centipede have to use a disguise spell (the third alien, a Human Alien, needs no disguise). The tree's disguise spell temporarily fails when he's trying out a baseball cap in a changing room, leading to a very startled store clerk.

Live Action TV

 * In Quantum Leap, Sam Beckett takes on the "Aura" of the person he has leaped into (see Voodoo Shark for a more thorough explanation.) Several episodes, such as the one where Sam leaped into a monkey, confirmed Sam is actually leaping with his own body, and merely appearing as the host to those around him. Of course, just as many episodes demonstrate the opposite to be true.
 * In one episode, Sam leaps into the body of a war vet who has lost both his legs. Al warns him not to try to get up. When Sam does get up at the end of the episode, the other person sees a guy with no legs floating in mid-air.
 * Kids, animals, crazies, and psychics can see his true self.
 * In Dead Like Me, the reapers look different to their true form to living people. We see "UnGeorge" in scenes through a living person's perspective.
 * In Big Bad Beetleborgs, this was combined with The Nth Doctor when Shannon Chandler left the series: Halfway through Season 1, Wolfgang, while playing with a magic book, accidentally caused Jo to change in appearance. To counteract this, Flabber used his magic to make her look and sound just like herself, except to the people who saw him do the spell. So the viewers, Flabber, Drew, and Roland could still see Jo in her new appearance, played by Brittany Konarzewski, but the Hillhurst monsters as well as their family and friends could see her as she looked before. The psychological impact this would have on her brother and best friend was never addressed.
 * Star Trek: The Original Series
 * In the episode "Spectre of the Gun", aliens put Kirk and company into a simulation of the gunfight at the O.K. Corral. They, and the audience, still see each other normally in their Starfleet uniforms, but the other people in the simulated town react to them as though they were Ike Clanton and company.
 * In "The Man Trap", the salt vampire could make itself appear to be a specific person to everyone looking at it. Because of his long experience with it, Professor Crater could always recognize it no matter what form it took.
 * The very special glamor spell used by The Charmed Ones in the early part of season 8 of Charmed allowed everyone who knows them well enough to see them as they are (most notably the child who couldn't recognize his own mother anymore otherwise) but everyone else will see the glamor.
 * Though they will see the glamor, and not themselves, in the mirror.
 * An episode of The X-Files follows this trope when Mulder accidentally switches minds with an employee of Area 51. Mulder, due to his career in weirdness, is able to BS his way through the other man's life fairly well.
 * In one episode of Legend of the Seeker, a witch casts a spell to make another man appear to be Richard, and vice versa. He doesn't realize it until they start treating him weird.
 * In Stargate SG-1, Reoul venom can do this. Daniel Jackson uses it to infiltrate a Goa'uld summit meeting at one point, masquerading as a System Lord's personal trusted slave. At one point he questions what happens if you use the venom on yourself. Selmak replies "I don't actually know, and that alone should scare you."

Tabletop Games

 * The Dungeons and Dragons spell "Disguise Self".
 * The "Illusion Disguise" spell in GURPS: Magic works like this. "Perfect Illusion Disguise" will actually fool all five senses.
 * In Changeling: The Lost, all changelings have been twisted and become inhuman, but normal humans instead see their mask, a slightly altered human guise. The changeling, and other changelings (as well as the Fey), however, are all capable of seeing the changeling's true form. The "Rites of Spring" sourcebook has a segment that explains that the Mask actually covers all aspects of the Mien (true form), but it is weakest in disguising scent and taste.
 * The "Familiar Stranger" power of Vampire: The Requiem's Obfuscate Discipline makes the target view the vampire as a person he knows. What the target perceives is not communicated to the player.
 * The Divine Fire hides a Promethean's true face. Given that Prometheans are animate corpses, this is pretty much the only thing that keeps them from getting killed on sight (and Disquiet means sometimes even that isn't enough). Prometheans can see each other's true faces, as well as how humans see them overlaid on it. There are, however, several ways to cause Glamour Failure, and those usually cause a world of problems...

Video Games

 * In the first Legacy of Kain game, Kain can get two spells that make him look like how he was when he was alive. These allow him to walk through towns without alerting the people.
 * This is how the Spy's disguises in Team Fortress 2 work, in practice. It's speculated that his disguise kit uses some form of holographic technology.
 * A spell that disguises Owyn as a moredhel in Betrayal at Krondor works like this.

Web Comics

 * In El Goonish Shive, Justin and Nanase can change their appearance through magical illusions.
 * In Eight Bit Theater, Red Mage casts an illusion spell on the Light Warriors that works this way. Since the change isn't immediately obvious, Black Mage spends a good deal of time mocking him for casting a spell that apparently did nothing before Red Mage gets a chance to explain.
 * Not a spell, but Freefall used a technological way to cause the same effect.

Web Original

 * Happened a few times during the Crossover Wars: during the Forum Wars the Evil Overlord forces attacked the Wotch forum disguised as members of the EGS & CRFH forums; during the Adult Wars Terry Dix cast a spell to make her & Miss Violence appear to be EOU droids & later convince the droids that M.Organ was Deception of the EOU.

Western Animation

 * In X-Men: Evolution, Nightcrawler uses a kind of hologram field device to appear as a normal human being—pointed out in an episode where he had to wear gloves to the prom to prevent his date from catching on to the fact that he had two fuzzy fingers.
 * He originally used one in the comics as well, but he mostly toyed with it to transform into famous movie stars or as a disguise. He stopped using it after a trip through space - mostly because Freaky Is Cool.
 * And because he felt it was contrary to the mutants-are-okay message.