Professional voice actors pride themselves on range. So, hiring a few good voice actors means you can take care of many, many characters with a small cast (especially if one or two actors are a Man of a Thousand Voices).
Oftentimes, this results in funny situations, like two characters played by the same person having intense conversations and heated arguments with each other. The talent is in making sure the audience doesn't know it. If jokes are made about this, it's Actor Allusion.
Sometimes this can happen in real life for technical or legal reasons where a person is required in one capacity to write themselves (in another capacity) a letter. For example, in most US states, for small corporations, a corporate officer can file an authorization to the board of directors to waive the corporation carrying workers' compensation insurance on that officer. Now, in the case of a corporation with a single owner (who therefore is both Chairman of the Board of Directors and President), the President of the corporation (who is the only officer) has to write a letter to himself, as Chairman of the Board, informing himself that he (as President) is waiving the Workers' Compensation coverage on himself (the letter being sent to the Workers' Compensation Board).
In voice acting, the process is fairly simple, with the actor just doing a different take (although some good voice actors can do it in real-time). The actor's vocal range is the only thing that might betray commonality.
This is sometimes actually invoked on purpose, as it can make you think, "Ohey, they're a clone? Why didn't I realize that before?".
In Live-Action this has historically been difficult, requiring split screen or otherwise splitting the image and requiring perfect synchronization between the different takes. Normally, the camera was stationary for this, but Back to the Future Part II pioneered a motion controlled camera that allows for complex panning shots that have the same actor in multiple roles. More modern works are able to do it a bit more easily due to advances in computer editing.
Not to be confused with Informing the Fourth Wall, Sounding It Out, Thinking Out Loud or Talking to Themself. Compare Holding Both Sides of the Conversation, which is an in-universe example of this trope, where a character is pretending to hold a conversation with another non-present (or non-existent) character, in order to maintain some kind of charade.
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- The original Chinese dub of Happy Heroes features the voice actors and actresses voicing quite a few main/significant characters at the same time:
- Liu Hongyun as Happy S., Careful S. post-Season 1, Doctor H. in Season 6, Headmaster Tele, Zelia in Season 1, and Ambassador Wang.
- Deng Yuting as Sweet S. for most of the series, Doctor H. for most of the series, Kalo, Lightbulb Jr., and Lele.
- Yan Yanzi as Smart S. and Little M.
- Zu Qing as Sweet S. in Season 6, Careless S., Careful S. in Season 1, Kalo in Season 6 and the first film, Miss Peach, the mailman robot, Zelia for most of the TV series, Keke, and Fire S. in the TV series.
- Li Tuan as Mr. Lightbulb, Teacher Know-All, Professor Limen, the Global Leader, Doctor H.'s father in the TV series, Planet Gray's Commander, Ambassador Miao, and Huo Haha.
- In Motu Patlu, Saurav Chakraborty provides the voice of both Motu and Patlu.
- In Pleasant Goat and Big Big Wolf, quite a few of the original Chinese actors voice more than one character at a time:
- Zu Liqing as Weslie, Night Wolf, and White Handsomey.
- Gao Quanshang as Slowy, Dark Handsomey, Master Paopao, and Brother Tai.
- Liang Ying as Paddi and Wilie.
- Deng Yuting as Tibbie, Jonie, and Fragrant Wolf.
- Liu Hongyun as Sparky and Banana Wolf.
- Zhang Lin as Wolffy and Yellow Wolf.
- Eddie Izzard:
- He does this on-stage, as would most stand-up comedians who do voices. However, he regularly mocks it too. Also, the only voices he can really do are Sean Connery and James Mason. Which he acknowledges as well.
- Particularly amusing is that it is Eddie Izzard playing James Mason playing God, scolding Eddie Izzard playing Sean Connery playing Noah.
- Jeff Dunham is not only an exemplary showcase of this trope as a ventriloquist, he's acknowledged it in his act through his puppet Peanut:
- Epically lampshaded by Peanut in Spark of Insanity, after Peanut jokes about the pronunciation of Jeff's name:
Peanut: You know, the weird part is I am actually pissing him off. And he would like to kill me! But he will not because that would be a form of suicide!
- Peanut also flat-out calls him on this during the appropriately-titled Arguing with Myself:
- Epically lampshaded by Peanut in Spark of Insanity, after Peanut jokes about the pronunciation of Jeff's name:
- Michael Mcintyre has been known to perform conversations with himself on stage, often adopting different voices while doing so.
Mcintyre: I've been down there and it's not pretty, they're all wearing trousers, so we're gonna open with a skirt. Modelling it here is Scott. You alright, Scott? I'm alright. But you've got me in a skirt. I'm not happy about that yet.
- In the comedy miming duo The Umbilical Brothers Shane often does all of the voices — including David's, who will often simply mime — and all of the sounds.
- ItsJustSomeRandomGuy does all of the male voices in I'm a Marvel... and I'm a DC. Mostly, he's very good at making each voice different — with the exception of the strangely gentle Captain America voice, they're all similar, but distinct. However, when characters are worked up they all sound the same.
- The internet radio drama Fobbies Are Borange, at one point, had a voice actor have a knife fight with himself.
- LittleKuriboh of Yu-Gi-Oh! The Abridged Series. Also by extension any Abridged Series. LittleKuriboh gets extra points though for doing a live-action reenactment of his first episode. Basically him being taped on a street in England switching voices/characters from one second to the next.
- Generally averted in the Phoenix Wright: Ace Attorney fandub of "Rise from the Ashes", with one notable exception: Phoenix and Edgeworth are the same actor. Of course, there is much shouting back-and-forth between them.
- Doubling as Cross-Dressing Voices, any speaking parts in the videos for Kill la Kill AU are all done by Amoridere.
- Random Assault: Whenever Mitch and Slabflapper talk to each other.
- A unique example of this happened in the WWF in 1997 when Hunter Hearst Helmsley was scheduled to face Dude Love in a Falls Count Anywhere match. Dude's music hit, but instead of coming out, he appeared on the Titantron screen and told Triple H he didn't feel like fighting the match and called in a replacement, who turned out to be Mankind, who proceeded to walk on screen and discuss the match with Dude Love. As if that wasn't enough, Mankind admitted that he had an even better substitute lined up, and brought on Mick Foley's original WCW and ECW character, Cactus Jack, leading to all three of the "Three Faces of Foley" sharing space on the Titantron at once before Jack came down to kick Triple H's ass.
- In any given Roleplaying game, the Game Master will, by necessity, be voicing all the NPCs. More talented or imaginative Game Masters will even give them distinct voices (which can get damn funny at times).
- Some campaigns have two Game Masters, averting the trope. On the other hand, in some games (such as Ars Magica) even the players will have several characters.
- In one hilarious game of Dungeons & Dragons, one player simultaneously played an elf and a dwarf who were Vitriolic Best Buds on the best of days. He used hand signals to indicate who was who whenever they got in an argument (again).
- Avenue Q features the same actress playing Kate Monster and Lucy the Slut, the same actor as Princeton and Rod, and the same actor as Nicky and Trekkie Monster. The conversations with themselves are particularly impressive because all the puppeteers are onstage and usually one of the puppets is being controlled by a completely different puppeteer who had to match the mouth movements to the other actor's words. Also, they sing too. Possibly lampshaded when you notice that the voice of Nicky/Trekkie Monster has the only puppets that require two people to work (can be used by one, but not as effective).
- Charles Ludlam's The Mystery of Irma Vep heavily invokes this trope (and occasionally lampshades it in the script). It's a show with four male characters and four female characters, with two male actors playing all of the roles. In particular, the actor playing Nicodemus and Enid not only holds conversations with himself during costume changes, but also plays out a scene involving playing the monster that's mauling Nicodemus off-stage.
- The whole point of rakugo theater: there is only one sitting performer on stage to act out the play, and the performer must convincingly switch between characters in the story and the narrator by way of body gestures, facial expressions and voice.
- This was commonly done in-story in Shakespeare's plays by his "clown" characters (played by comic actors who were presumably able to do multiple voices):
- Feste in Twelfth Night stands outside the dark cell in which Malvolio is imprisoned and enacts a conversation between himself (Feste) and "Sir Topas", the minister supposedly sent to "cure" Malvolio of his insanity. Malvolio falls for it, of course.
- Launce in Two Gentlemen of Verona performs the scene of his departure from home for the audience, using a pair of shoes as his parents, his staff as his sister, his dog as himself, and himself as his dog.
- Launcelot Gobbo in The Merchant of Venice parodies the conventions of a medieval morality play by playing himself, his conscience, and the "fiend" who wants him to run away from his master.
- In A Midsummer Night's Dream Nick Bottom begs to play all the parts in the play-within-a-play, and acts out a scene between star-crossed lovers Pyramus and Thisbe before roaring to represent "the lion."
- In The Wizard of Oz On Ice, Bobby McFerrin voiced all of the characters except for Dorothy. (Yep, even Toto.) In the TV special, he also voices Dorothy.
- In 1600 Pennsylvania Avenue (and the concert redux, A White House Cantata), one actor plays every President and one actress plays every First Lady. This makes "Duet for One" interesting, where the actress has to switch back and forth between Julia Grant and Lucy Hayes.
- The Comedy About A Bank Robbery features a three-way argument between characters played by the same actor. In addition, there are no props and everything is mimed. It gives the actor a great chance to show off his versatility.
- In the Pirates of the Caribbean attraction during "A Pirate's Life For Me", Thurl Ravenscroft voices a drunk pirate singing the song and his dog howling alongside him.
- As mentioned above, Game Masters do this often, so it's no surprise that the GM of Darths & Droids does this as a lot. There was also that one time when Jim was given control of his character's family, and found himself in a situation where he had to roleplay girl talk with himself.
- In the Watchmen motion comic, ALL the characters are voiced by one guy.