The character you know and love walks off set on one side, and a couple of seconds later walks in on the other side, only he's wearing different clothes! And talking funny! And everyone's calling him Cousin Rick, not Fred!
For many a reason both solid and sordid, an actor might find themselves playing more than one role on the same show. It might be a twin brother (or cousin, aunt, etc. — television has never been fussy on the details). A male character may be put in drag to play his own mother, who looks a lot like him. More than a few action shows have had a lookalike try to frame the main character. Whatever the reason, the actor is Acting for Two. Sometimes more, depending on the role.
It happens occasionally in other media as well, but when the same actor plays multiple characters on TV or in a film, it usually has a very specific purpose. In theatre, it's just as often an economic use of talent. Often certain role-pairings become traditional, so for example some film versions of Peter Pan still cast the same actor for Hook and Mr Darling - even though they could afford two actors, and the stage tradition only arose because of their lack of scenes together. Maybe because it seems symbolic of... something.
This tradition goes way way back. Classical Greek drama was usually performed by two or three actors (wearing masks) plus chorus. As a result, you never see more than three principal characters at the same time, in very specific combinations: Hecuba talks to Cassandra and a Greek Herald, then Cassandra goes offstage (and changes masks) and comes back on as Andromache, then Andromache and the Herald go offstage and come back as Helen and Menelaus... To this day "doubling" of roles is common in theatre, especially in No Budget productions.
This particular little ice cream cone comes in several flavors, depending on the purpose, and varying in utility by medium:
- And You Were There: Like a Mirror Universe, but with a fun-house mirror (think "Wizard of Oz").
- But You Were There, and You, and You: A character tells a story, and the characters are depicted as people the storyteller knows.
- Cast as a Mask: An inversion. Different actors play a character's different identities.
- Cloning Blues: Well, technically, a clone is related to you...
- Doppelgänger: An in-story reason for them to be played by the same actor.
- Doppelgänger Replacement Love Interest: Hooking up with someone identical to your lost love.
- Dream Sequence: Not quite the same character, but pretty close.
- Ghost in the Machine: When "the little guy in your head" looks just like you.
- Good Angel, Bad Angel: When the angels on your shoulder look just like you.
- Identical Grandson: A character's descendants are played by the same actor.
- Identical Stranger: A new character arrives who looks just like an existing character.
- Inexplicably Identical Individuals: When multiple, nominally unrelated characters are all identical.
- Latex Perfection: Different character, same actor, because character has a perfect disguise.
- Loads and Loads of Characters: If you don't have loads and loads of actors to go with them, you get this problem.
- Lost in Character: A character who is an actor gets so into character they basically become a new character.
- Mirror Universe: When everybody has their counterpart, you can have twice as many characters per actor.
- Playing Their Own Twin: Characters who are identical twins (or triplets, quadruplets, etc.). Why hire two actors when one can do the job?
- Reincarnation: In the "looks just like the old me" variant.
- Surgical Impersonation: An actor plays two characters, one of whom changes his face surgically to resemble the other.
- Talking to Himself: Two (or more) characters voiced by the same voice actor, in a conversation with each other.
- Talking to Themself: When two or more parts of a Split Personality engage in conversation.
- Uncanny Family Resemblance: Any family relation is played by the same actor.
- You Look Familiar: One actor plays two unrelated characters, within the same series, but (usually) different episodes.
See Double Vision for a look at how they manage the trick of getting an actor on-screen more than once, when needed.
Understandably this happens a lot in animation, simply by giving the same voice actor multiple roles; see Talking to Himself for that version. Not to be confused with Talking to Themself, in which the actor plays different personalities of a single character in-story. For the other kind of "acting for two," see Hide Your Pregnancy.