Loads and Loads of Roles

There's Acting for Two. Then there's acting for twenty.

This is where one actor plays a considerable number of roles in a single work (we are not counting Sketch shows)

This is primarily a theatre trope, where it requires the ability to perform pretty rapid costume changes.

For the animated equivalent, see Talking to Himself. You Look Familiar is this trope carried over the life of a TV Series. You All Look Familiar is the video game character model equivalent.

Film

 * Kind Hearts and Coronets. Alec Guinness plays about eight different characters.
 * A BBC Radio remake had Harry Enfield reprising all of Guinness's roles.
 * A good many Peter Sellers films, especially Doctor Strangelove, where Sellers plays the title character, Gen. Ripper's British adjutant and the US president. He was originally intended to play the bomber pilot as well, but he A) allegedly had a dubious time with the accent and/or B) sprained an ankle and could not work in the cramped cockpit set, so the role went to Slim Pickens. Columbia Pictures only agreed to finance the film on condition that Sellers play multiple roles.
 * In The Mouse That Roared, Sellers had three separate roles - as Tully; as Lord Mountjoy; and as Grand Duchess Gloriana of Grand Fenwick. This was explained (and Lampshaded) by the disclaimer at the beginning of the movie that the First Grand Duke was "in every possible sense, the Father of His Country" (and indeed, his memorial statue looks exactly like Peter Sellers).
 * The biopic The Life and Death of Peter Sellers has Geoffrey Rush not only playing Sellers and his characters, but also suggests Sellers is directing it and playing everyone else in the film by sometimes stopping the action and revealing, say, Blake Edwards (John Lithgow) to be played by Rush-as-Sellers, and commenting to the viewer on Sellers' behavior. This may be a confusing attempt at a Deconstruction of Sellers, given the film's overall attitude towards him as a person and errors that can be seen as They Just Didn't Care for those familiar with his life and work.
 * Tim Burton's film of Charlie and The Chocolate Factory, where one actor (Deep Roy) plays all the Oompa-Loompas through the miracle of CGI.
 * Any Monty Python film, for all six of the Pythons involved--although Graham Chapman usually had relatively few roles, due to being the relative Straight Man and thus usually playing the central characters.
 * Mike Myers playing the role of several key characters in the Austin Powers franchise.
 * In the first film he played only Austin and Dr. Evil, but picked up the roles of Fat Bastard and Goldmember in the sequels.
 * In Coming to America, Eddie Murphy and Arsenio Hall play several different roles, including white or female characters.
 * Eddie Murphy is doing this a lot more nowadays.
 * The League of Gentlemen, in which, as the ads put it, "three guys don the shoes and dresses of an entire town."
 * Jack Black's character from Tropic Thunder made a career out of this.
 * In one surprisingly inspired scene Robert Downey Jr.'s character has an emotional breakdown that causes him to progressively remove his make up. At each stage he takes on a different accent and personality.
 * In the short film Ghosts, Michael Jackson plays two main roles (the Maestro and the Mayor) and three transformed versions of them (Skeleton, Superghoul and Ghoul Mayor).
 * In Clerks, several actors play multiple small roles. Given the late hours and low pay, many of those cast simply failed to show up, so Smith would have anyone who happened to be around step in. David Klein (the cinematographer) and Walter Flanagan get hit the hardest with this, with five and four roles respectively.
 * Buster Keaton's silent comedy The Playhouse (1921) opens with a scene in which every character at a theatrical performance - actors, stagehands, musicians, audience members, a twelve-man minstrel show - is played by Keaton, impressive both for Keaton's skill and for the technical aspects of pulling something like this off so early in film history. It's even lampshaded when one of the Keatons remarks to another, "This fellow Keaton seems to be the whole show."
 * The Wizard of Oz (1939) had Frank Morgan playing five different roles: The Wizard, Mr. Marvel, a gatekeeper in Emerald City, a carriage driver, and a guard in the Wizard's palace.
 * The 1971 adaptation of Neil Simon's Plaza Suite had Walter Matthau playing a different character in each of the film's three acts.
 * The original Broadway production had George C. Scott and Maureen Stapleton appearing as different characters in all three acts, while a 1987 made-for-TV version had Carol Burnett playing three different roles.
 * Fred Astaire does this in The Man In The Santa Claus Suit. (Although technically )
 * In the Japanese VHS/DVD dub of Forrest Gump, Katsuhisa Hoki voices over the Doctor, Alabama Governor George Wallace and President Lyndon Johnson.
 * Tony Randall in "The Seven Faces of Dr. Lao" plays Dr. Lao, various acts in his circus, and a brief moment as a spectator.

Live Action Television

 * Irish comedy Killinascully has its writer Pat Shortt play five major characters (one of whom is female), frequently appearing in the same scene as himself.
 * There are five actors who have played seven or more different characters over the course of various Star Treks: Vaughn Armstrong, Thomas Kopache, Jeffrey Combs, J.G. Hertzler, and Randy Oglesby. Most of them started out playing a series of one-shot characters, then were given frequent recurring characters of their own. Jeffrey Combs was even supposedly going to make the jump to actual cast member, but Enterprise was canceled a season too soon.
 * Honorable mention to Majel Barrett for playing several major characters: Number One (original pilot), Nurse Christine Chapel (Original Series), Lwaxana Troi (Next Generation), and Federation starship computers from TNG onward, including Enterprise (one-episode cameo) and the latest movie. This makes her the only person to be involved in every incarnation of the franchise. Of course, she was Gene Rodenberry's wife.
 * Technically, Robert Picardo should be on that list. He has played dozens of different incarnations of the Emergency Medical Hologram. (Identical in that they have the original base program, but different in that they have had different life experiences, if only subtly.) Not to mention playing the designer of the EMH, and a few holograms that aren't EMHs, but look like the EMH.
 * And then there's Brent Spiner, who's played three different Soong-type androids (Data, Lore, and B-4) and two different Soongs (Noonien and Arik). The crowning achievement is in the episode "Brothers", where he plays Data, Lore, and Noonien Soong in the same scene at the same time (with the help of Chroma Key, of course).
 * On The Mighty Boosh, dozens of prominent - and minor - characters are played by Noel Fielding, Julian Barratt and Rich Fulcher. Fielding (as well as playing the major character of Vince Noir) fills notable roles such as the Hitcher, the Moon, Old Gregg, Tony Harrison, Montgomery Flange, the Spirit of Jazz and Spider Dijon. Barratt (who plays Howard Moon) also plays Rudy van DiSarzio, the Crack Fox, Mr Susan, Dennis the Shaman, Bryan Ferry, Sandstorm, and Jurgen Haabermaaster. Rich Fulcher, who is best known for playing Bob Fossil, has given many other very memorable performances over the course of the series, including Lester Corncrake, Kodiak Jack, Tommy the Cheese Priest, the Ape of Death, The Betamax Bandit, Alan (the Blue King of Xooberon) and "Eleanor", among others.
 * Dave Brown really deserves a mention too. Although his only recurring roles are Bollo the gorilla and Joey Moose, he plays a fair few minor parts - Black Frost, the break-dancing Mutant, Naan Bread, Extreme Sports Calender etc.
 * Before becoming famous with The Red Green Show, Steve Smith and his wife Morag produced a Canadian sketch comedy show called Smith and Smith, which aired in the late 1970s and early 1980s. Due to the show's very low budget, among other reasons, Steve and Morag played the majority of the roles themselves.
 * Peter Kay in Phoenix Nights
 * Mega 64. The six main characters are played by three actors (two characters each), and there are a few regular recurring actors who almost always play different characters in each appearance.
 * In the Japanese Nippon TV dub of the Twilight saga, Kenji Nojima voices over Jasper Hail, Caius and Jared.
 * Dick and Dom In Da Bungalow: the multi-talented Dave Chapman and Ian Kirkby played tens of characters each- all kinds of impressions (Murray Walker, Bruce Forsyth, Richard O'Brien, Eric Morecambe...) and also original creations like Harry Batt. Melvin O'Doom had slightly fewer characters, but was also an all-purpose dancer. Even the hosts also played several recurring charaters each.
 * In Jul På Vesterbro, Anders Matthesen plays every single character who has talking roles in more than one episode. In other words, he plays 11 main and recurring characters, 3 one/two episode characters as well a cameo As Himself.

Tabletop RPG

 * If your Tabletop RPG has a lot of NPCs in it, your Game Master will have to either enlist help playing them all, or else just play them all himself.
 * With each member having access to at least four characters in Marvels RPG, odds are that they will end up with this.

Theatre
"Big Bad: *is shot, with Hannay, a police officer and the girl looking on* There was only supposed to be a cast of four!"
 * This trope taken to the extreme can be found in the theatrical adaptation of the Alfred Hitchcock The Thirty Nine Steps. The show has over 100 roles. There are four actors.


 * The lead man plays one role, the lead woman plays four, and the remaining two actors play all of the other roles between the two of them. Hilarity Ensues, as does much switching of hats, when four or five of these characters are in the same scene. It prides itself in Lampshading everything about this trope.
 * Another impressive example is The Laramie Project, wherein eight actors play over seventy different roles total.
 * Angels in America, which is also written to be performed by eight actors, specifically states in its dramatis personae that certain lead characters double for certain supporting characters, with the actresses playing Hannah and the Angel with the highest total role count. Memorably, Meryl Streep in the television adaptation.
 * Patrick Stewart once did a one-man version of A Christmas Carol. No costume changes of course.
 * In the 2009 CGI film, Jim Carrey plays no less than eight roles, including all ages of Scrooge and all three spirits. In this case, it overlaps with Talking to Himself.
 * In the 1970 musical film, Albert Finney also played the adolescent/young adult Scrooge.
 * ACT Theatre's annual production also does this, with some actors playing up to four or five roles.
 * The various Greater Tuna plays have most or maybe even all of the inhabitants of a very small (fictional) Texas town played by only two guys.
 * Every performance by the Reduced Shakespeare Company, whose plays consist of three actors performing "abridged" versions of large bodies of work.
 * The play Stones in His Pockets, where two people played every role. With a few boxes as their only props.
 * The Broadway version of Rent has very distinctive-looking actors that together play all of the extras. In some scenes (like "Christmas Bells") the costume changes are fast enough and thorough enough to make it look like there are three times as many of them as there actually are.
 * I Am My Own Wife is a one-man play with the sole actor playing more than forty different roles. However, many of them have only a few lines, and there are no costume changes--all of them are performed in the main character's (Charlotte von Mahlsdorf) black dress and pearls.
 * At one point, a production of The Importance of Being Earnest was staged with two actors. In the final scenes, they resorted to wearing both dresses and suits while holding umbrellas: they'd open the umbrellas to block the suits when playing women, and close them again when playing men.
 * Anna Deveare Smith does this for the play/film Twilight Los Angeles.
 * The Broadway musical The 25th Annual Putnam County Spelling Bee has only nine actors in its entirety. The nine actors each have a main role, and double for 13 others. Often, the actors assume their other roles by unzipping vests, moving their hair, or merely changing their voices.
 * The Mystery of Irma Vep (not to be confused with the film Irma Vep): two actors (who are contractually obligated to be the same sex to insure that there will be crossdressing), 8 characters, 60 quick costume changes each in the course of a show that only runs 100 minutes. Anyone who has worked on this show will freely admit that the dressers for the actors are as vital to success as the actors themselves.
 * The play A Dream Play (originally Ett Drömspel) by Swedish playwright August Strindberg is, as the title suggests, All Just a Dream. As such, nobody reacts to the fact that several characters are played by the same few actors, because it makes perfect sense in the dream.
 * A recent US tour of Chicago had a full ensemble, and the entire cast did one role apiece - except for one guy who played all twelve members of the jury. By taking little things out of his jacket, changing his posture, and not saying a word, he completely stole the climactic trial scene.
 * The Canadian classic Billy Bishop Goes To War. One actor, eight characters, not including the "narrator" who also provides musical accompaniment.
 * There is a stage version of Around the World In Eighty Days that was written for four men and one woman. The official character count is 39. "Actor 5" only plays Phileas Fogg, but his lack of roles is made up for by "Actor 1," who plays 16.
 * Another Neil Simon play, the musical adaptation of the Patrick Dennis novel Little Me, was produced on Broadway in 1962 with Sid Caesar playing numerous characters.
 * Subverted in Seussical. The Cat in the Hat appears as different roles throughout the show, but he's still The Cat in the Hat.
 * Book-It's 2011 play of Great Expectations had but 9 actor (save for a couple uncredited stage hands), most of them playing dual roles, in addition to minor characters/extras. Even better was their play of The Secret Garden, which used only three actors, with several cross cast roles.
 * In the Hugh Wheeler version of Candide, the actor playing the narrator (Voltaire) also plays Pangloss and a few other characters, and has to perform quick costume changes with no time to duck offstage.
 * All minor characters in Little Shop of Horrors are played by the same actor, according to the official script.
 * In First Date, the four ensemble actors, as well as the waiter, play all of the Flash Back characters, with on-stage costume changes.

Web Original

 * Quite common in That Guy With the Glasses.
 * Doug Walker is, among others, the The Nostalgia Critic, Chester A Bum, Ask That Guy With the Glasses, Dominic, Douchey McNitpick, Raoul Puke, General Zod, Dr. Smith, and Tommy Wiseau.
 * Linkara also plays Mechakara, 90s Kid, Ensign Munroe, Dr. Linksano, Harvey Finevoice and the Ninja-Style Dancer.
 * Noah Antwiler/Spoony plays *inhales* Dr. Insano, Chef Ramsay, Squall Leonhart, Fu Manchu, Spencer D. Bum, the Ultimate Spoony, Cap'n Smiling Jack D'Arcy, the Gate Cleaner, Hulk Spogan, Terl, "Spoonette", Spoontock! of the Klingon Empire, Chuckles the Fucking Jester, Tidus, and The Goddamn Avatar.
 * The three cross-over Warrior reviews between Atop the Fourth Wall and The Spoony Experiment all involve shifts between various versions of Linkara and Spoony for most scene shifts in the film footage. Some of those alternate versions are played by other actors. Many aren't.
 * Brad Jones is The Cinema Snob, Kung Tai Ted, Vic Dealio of "The Big Box" and Eighties Dan.
 * The Chestnut Hill Improv Group is only three people, two of whom do a lot of voices between them.