Master Actor

The Master of Disguise is a character who can slip in and out of costume at the drop of a hat, who is such an accomplished actor and makeup artist that no one can identify them until they reveal themselves.

This trope is just about the "accomplished actor" part.

When they want to, they can change their mannerisms, their voice, and every other non-physical aspect to seem like a completely different person. Usually done by evil characters, in a Bitch in Sheep's Clothing sort of way. This is the kind of character that causes fans to cry "wait, is that even the same character?"

A Required Secondary Power for most Shapeshifters. Otherwise, how could they pass as - for example - the President of the United States? Of course, much humor can be had if they don't fall under this trope.

A sub-trope of Master of Disguise. An in-universe version of He Really Can Act. Can overlap with The Power of Acting. Contrast Hugh Mann, for when a shapeshifter doesn't get this as a secondary power.

Anime

 * in Kämpfer is secretly the Big Bad and behind nearly everything in the series--including, quite possibly, the main character's stupidity. Very few people saw this coming, and those that did assumed it would be a case of Split Personality.
 * Captain Aizen in Bleach to the point it's no longer a spoiler. Also, to a lesser degree his subordinate Gin
 * Airi in the anime Those Who Hunt Elves is a supreme actress; in addition to being a master of disguise, she can manipulate just about anyone with the right act.

Film
""He'll have to act fast!""
 * Superman. Christopher Reeve was such an accomplished actor that he could make it plausible that people who knew both Superman and Clark Kent wouldn't realize they were the same person.
 * Gary Johnston is recruited for Team America World Police specifically because the team needs a brilliant actor. When he leaves the team, they're destroyed in their next fight: "Without an actor, they were like pigs to the slaughter." At the end, he saves the day with his acting.

Literature

 * Kellhus in The Second Apocalypse is a man of near-god level intellect and virtually zero emotion. However, he has complete control over his entire body, including facial features and voice, so he can fake emotions very well. When someone figures it out and confronts him, it's mentioned that every muscle in his face just goes dead, like a switch was flipped.
 * Robert Heinlein's Double Star. Lorenzo Smythe is shown in the novel to have this ability, to the point that someone who actually knows him doesn't recognize him because of his acting ability.
 * The character Varys is a master of disguise in George R. R. Martin's A Song of Ice and Fire, and uses this skill in the course of his duties as the king's spymaster.
 * Silk of The Belgariad is shown to do this multiple times in the series through a combination of acting talent and contorting his face to change its apparent shape.
 * from Harry Potter.
 * Also  He may have   but keeping the act up for most of a year would require considerable skill and focus . Even more so in the film continuity where
 * Sherlock Holmes.
 * Ghost in the Tokaido Inn, one of Thomas and Dorothy Hoobler's Judge Ooka mysteries, has Tominio, a Kabuki actor whose ability to impersonate women is eerie, and also a Chekhov'sGun.
 * In The Shakespeare Stealer, it is revealed that Simon Bass, the Big Bad, is a former member of Shakespeare's troupe, and a gifted actor, leading to the revelation that.
 * Players of Gor. Tarl falls in with a Commedia Dell Arte troupe, one of which is a classicly trained actor who never appears on stage. Then in the book's climax he saves Tarl's life by acting the part of an Imperious General, much to Tarl's surprise, who says "You can act!."
 * Jack from the Dragonback books. When he does a radio response pretending to be a nasty mercenary called Chiggers, his method acting was so perfect that to Draycos, he looked like he had flipped a switch and somehow become the guy.
 * Tomjon from the Discworld book Wyrd Sisters, as the somewhat accidental result of the gifts given to him by the Witches.
 * in Dark Jenny. Up until the climax she's mentioned but apparently never seen, until it's revealed that she's a master of disguise who's shown up as at least four different characters. With no makeup or other external aids, though it's unclear whether magic is involved.
 * Fisk, from the Knight and Rogue Series. This is a result of much practice, since he's a professional con man.

Live Action TV

 * Saturday Night Live: Jon Lovitz's "Master Thespian," if only in his own mind.
 * Burn Notice: Michael Westen, Fiona, and a few other characters.
 * The Pretender was based on this. Jarod was taken from his family at an early age because he showed potential for being able to slip into the identities of others unnoticed. After escaping from The Centre, he used his talents to help people in need.
 * Several characters in Mission Impossible, particularly Rollin (Martin Landau), Cinnamon (Barbara Bain), and Paris (Leonard Nimoy).

Video Games

 * The World Ends With You: Joshua is a supreme example, considering
 * Going on WMG, of Umineko no Naku Koro Ni might be this. Throughout the sixth arc, he acts like an incompetent mess,  But a WMG theorizes that, which if true means that he managed to not only fool , he managed to fool , most of whom thought he was just being an idiot.

Webcomics

 * of Girl Genius takes this to truly impressive levels, along with Obfuscating Stupidity, fooling everyone (especially the audience) into severely underestimating her. Repeatedly. Even after being warned not to. Even after warning each other not to. Even against the Big Bad.
 * This is also an Informed Ability of the Lucrezia Mongfish, though the acting we see her do is rather unimpressive (granted, she knew nothing about ).

Western Animation

 * Zarana from G.I. Joe. Her brother Zartan (who actually came first) as well; he is such a master of disguise and language that he is able to perfectly copy someone's voice after hearing them speak once.
 * Samurai Jack: Aku, being a shapeshifter, is good at this as a Required Secondary Power. He once traveled with Jack for a good period of time without getting found out, while in the form of a humanoid woman.