A minor character or group of minor characters who offer commentary and/or opinions on the actions of the main characters, usually by Breaking the Fourth Wall and addressing the audience directly. Often, they say what the audience thinks (or should think). While a lead character can do this himself, it doesn't make him a Greek Chorus; a proper Greek Chorus differs by being removed from the action and thus able to view it with something approaching objectivity. The role is frequently played by Those Two Guys. It's one use for the First-Person Peripheral Narrator.
Strictly speaking, an omniscient narrator usually wouldn't qualify as a Greek Chorus. However, the lemony type who repeatedly breaks the Fourth Wall and makes asides to the audience to the point that they're a "character" unto themselves might reach the point where they overlap with it. If the narration is revealed to be by an actual main member, retroactively telling the story to someone else, it may count, but the important qualifier is that their opinions are objective and express what the audience would think (if they are retroactively self-deprecating of even their own actions, etc.).
Named for the choruses of ancient Greek theatre, who did exactly this. Aristotle discussed them in Poetics and warned that they should be used as little as possible, because (in modern paraphrase), what they do is, by definition, commentary, not story — and so the story-teller should avoid being sidetracked by using more than it needed to help the story.
The Snark Knight, if not the main character, often fits this role. See also Fourth Wall Observer.
Compare this to Mystery Science Theater 3000, where the comments come from outside the story.
The Durarara!! anime features segments with online chatrooms with unidentified people casually discussing various events and rumors related to the plot. This seems to function as a Greek Chorus, except for the fact that over time, the audience will realize that several main characters are the participants in these conversations, and some of the conversations are spoken in deliberately misleading voices to keep you guessing as to who's who.
A debatable example is Neya from Infinite Ryvius: For most of the show she is mostly there to echo and accent the feelings of the cast.
"Babbit" from Kodomo no Omocha is a Greek Chorus. Whether or not it is acknowledged by other characters depends on the situation (it seems to not really exist in serious situations, but may be noticed and perhaps smacked in sillier situations).
The Japanese senior citizens (identified in the anime as the town council elders) in the town around the Hinata Apartments act as a Greek Chorus in Love Hina. However, their pronouncements can be heard by any character who is close by, and they even give Keitaro a hard time once by keeping his sketchbook away from him, Monkey In The Middle style.
In the anime Revolutionary Girl Utena, the Kashira Shadow Players (or Shadow Play Girls) act as a Greek Chorus. Their shadow plays usually parallel the events of the episode, with varying degrees of subtlety. Once, though, they actually invite some characters (Utena, Anthy, and Akio) to see one of their plays, a subtle-as-a-kick-to-the-head story about Anthy and Akio's past. Additionally, when the shadow play (each of which ends with a question) is performed near Utena, Utena answers the question that is posed at the end.
Fuu and Ryou from Sketchbook often fulfill this role, although their comments don't always seem to have any direct bearing on the show — which isn't helped by the fact that they're Cloudcuckoolanders..
Alpha Q is a Greek Chorus of one after his death in Transformers Energon. He keeps assuring the audience at the beginning that something cool will happen. Something cool does indeed happen: the end of the series.
The (usually civilian) supporting cast of a superhero title are usually this trope, sometimes extended to the superhero's hometown. For example, Lois Lane, Perry White, and Jimmy Olsen (and frequently the rest of Metropolis) for Superman, the Gotham police department for Batman, the staff of the Daily Bugle for Spider-Man, etc.
The Little Archie story "The Long Walk" gives this role to three of Betty's toys: a naive panda teddy, a conceited sailor figure, and a wise witch doll.
The film Mighty Aphrodite, despite being set in the present, features an actual ancient Greek chorus that the main character has conversations with. This may be the only time you'll ever hear an ancient Greek chorus shouting, "Don't be a shmuck!"
Not Another Teen Movie featured various random Genre Savvy characters who made snarky comments in regards to the trite teen movie cliches/conventions the main characters were expressing.
In Stardust, the princes who were slain linger behind as ghosts, unable to pass on to the afterlife until the next heir to the throne is found. Being unable to interact with the physical world, either, they can only observe and comment on whatever unfolds before them.
Senor Love Daddy and the three people sitting across from the Korean grocer in Do the Right Thing.
The slugs in Flushed Away dabble with this. Most of the songs they perform are reminiscent of what's going on in the story at that moment, to the point that it starts to feel like narration. For example, they sing a parody of "Bella Notte" in a scene where two characters are falling in love.
The Narrator fulfills this role in The Dark Tower, except for the parts where he actually appears in the book.
Illium by Dan Simmonsplays with this in its intro. The story begins with the narrator laying out the basics of the story in dramatic fashion, much in the same style as an actual Greek Chorus ("Sing, O Muse..."). Then it's revealed that the narrator is, in fact, the main character speaking in the first person and that he has, in fact, been resurrected from the dead specifically to tell you the story. He actually lampshades the trope:
If I am to be the unwilling Chorus of this tale, then I can start the story anywhere I choose. I choose to start it here. [And the plot begins]
Buffy's Andrew filled this role in "Storyteller". However, like most tropes in the show's later days, it was pretty ruthlessly deconstructed: Andrew realized he was putting a rhetorical spin on death and suffering for the sake of his own vanity (he's talking to a camcorder throughout the episode).
A Bound and Gagged Lorne found himself in this position on Angel's "Spin the Bottle". The events are being retold to an actual audience, so we know he gets free at some point, but the slowness of his rescue is commented on frequently. At the end of the episode we see that the whole time the bar was totally empty. Mind Screw, anyone?
Lorne: (still tied up) "I know I'm supposed to be unconscious right now, but can you believe these mooks?"
Jimmy Olsen and Perry White fill this role in Lois And Clark: The New Adventures of Superman, spending undue amounts of time commenting on the titular power couple. They even lampshaded this in one episode, when Jimmy complains about never having their own plotlines — only for the scene to instantly switch back to Lois and Clark.
In the First Doctor story/The Gunfighters called/A ballad kept track of/Events to befall/Lynda Baron performed it/Tristram Cary wrote the tune/And the song was called "The Ballad/Of the Last Chance Saloon".
In the later serial Vengeance on Varos, this role falls to married couple Arak and Etta, who are watching the events of the episode on television.
Hurley, Miles, and Frank Lapidus act like this in the later seasons of LOST.
On Friends, Phoebe's guitar songs often take this form.
Early in the fourth season of Hannah Montana, Rico had one in the form of a gospel choir that followed him around and sang about his words and actions.
Future!Ted in How I Met Your Mother often qualifies, providing a great deal of snarky, hindsight-enhanced commentary on his and his friends' actions. He often freezes the entire universe so that he can basically say "lol no" whenever a character makes an inaccurate prediction or does something that is going to eventually bite them in the ass.
Suzuki St. Pierre, the recurring comedic news reporter in Ugly Betty.
In Professional Wrestling the color commentators serve as the Greek Chorus, modeled after real Sports Casters, but also commenting on the drama and story-lines, and sometimes being made a part of them.
Traditionally there is a Face commentator and a Heel commentator, if there are 3 rather 2 it's always 2 Faces. Sometimes one of the active wrestlers will join for one match, involving a rival he's in an angle with.
Spiritualized's "I Think I'm In Love" brings in a gospel choir to comment on J Spaceman's attempts at positive thinking.
I think I'm in love (Probably just hungry)
I think I'm your friend (Probably just lonely)
I think you got me in a spin now (Probably just turning)
I think I'm a fool for you babe (Probably just yearning)
I think I can rock and roll (Probably just twisting)
I think I wanna tell the world (Probably ain't listening)
I think I can fly (Probably just falling)
I think I'm the life and soul (Probably just snorting)
I think I can hit the mark (Probably just aiming)
I think my name is on your lips (Probably complaining)
In the tragedies of Aeschylus, Sophocles and Euripides, the choruses usually represent women or old men who do nothing except comment on what's happening. There are exceptions to this, especially The Suppliants by Aeschylus, in which the chorus represents the protagonists, and The Eumenides, also by Aeschylus, in which the chorus represents the antagonists.
Many of the comedies of Aristophanes are named after the roles played by chorus: The Frogs, The Birds, The Wasps, etc.
Aristole complained about the later ones in Poetics, though, pointing out that the chorus was becoming more and more detached from the plays, and so less and less commentary on them.
Inherit the Wind has the reporter E. K. Hornbeck providing commentary... in verse.
Crystal, Chiffon and Ronnette in Little Shop of Horrors are an interesting example, going back and forth between standing outside the action and commenting on it to the audience, and interacting with the other cast members using no special out-of-character knowledge. You can keep track by the costumes; when they're characters the girls wear worn-down clothing appropriate to residents of Skid Row, and when they're a Greek Chorus they've changed into sparkly dresses.
The Rodgers and Hammerstein musical Allegro relies heavily on its Greek Chorus to voice characters' thoughts, particularly during the protagonist's childhood when he is neither seen nor heard.
In Mamma Mia!, this trope is taken literally: citizens of the Greek island where the main characters live often provide a chorus for the songs. They also make their own opinions on the action obvious on occasion.
Legally Blonde, Elle's sorority sisters, the Delta Nu girls. "Margot, Serena, Pilar? What are you doing here?" "This (indicating Elle's ex with a new girlfriend) is a tragedy. And every tragedy needs a Greek chorus!"
The Love Of The Nightingale has two, a male and female. The male chorus are the ones recounting the plot. The female chorus is Procne's tribal women companions who warn her of danger.
Britten's The Rape Of Lucretia has a Male and a Female chorus - a tenor and a soprano. Kinda one-person choruses...
Spider-Man: Turn Off the Dark has a "Geek Chorus" of comic book nerds in Version 1.0. They were dropped for the rewritten version that actually opened on Broadway.
Those Two Guys Salarino (or Salerio) and Solanio in The Merchant of Venice extract plot details from other characters and discuss plot-relevant offstage happenings (Bassanio's departure, Shylock's attempts to take legal action, etc).
Rusty, Urleen and Wendy Jo in Footloose. Rusty is less detached, being a character from the original movie (and part of the Beta Couple), but Urleen and Wendy Jo were written into the musical version to provide both this trope and 80's-style backup singing. (Wendy Jo also appeared in the movie.)
Lazarus and Eliza in Deus Ex: Human Revolution. Eliza is a mainstream newscaster, whereas Lazarus is a radio talk show host who deals with conspiracy theories. Usually more often than not, their broadcasts will deal with the current situation at hand with the player.
The Star Fox (and Wolf) pilots in Super Smash Bros. Brawl. Snake's allies from the Metal Gear series arguably count, but differ slightly, in that Snake himself is contributing to the discussion while he fights.
In the DS version of Disgaea, once you win (or get a Non-Standard Game Over), an option appears to turn on a prinny commentator, who often makes snarky comments on the game's goings-on.
In Mass Effect 3, there are Privates Westmoreland and Campbell, the two female Normandy crewmembers guarding the War-Room, who often comment about the events in the story as they occur.
By the 7th Arc of Umineko no Naku Koro ni, there are two pairs of these: Featherine and Ange in Dawn, and Furfur and Zepar in Requiem.
Ultra-annoying side characters Peeper and Greasy, at the Super Hero School Whateley Academy in the Whateley Universe. They even run a semi-legal campus radio station to broadcast their usually-offensive thoughts, and they provide the running commentary for the battles during the end-of-term Combat Finals. Unfortunately, they're utterly focused on the breasts of the hotter girls on campus, with the protagonists being some of their favorite targets.
In an episode of Phineas and Ferb, the band "Love Handel" follows a delivery driver around, singing songs about everything he's doing, and near the end of the episode, they reference their transformation from "a pop metal band into a rousing Greek Chorus".
Dmitri and Sviatoslav, the clock bats in Count Duckula would often comment on the proceedings and make terrible jokes on the theme.
The Badly-Drawn Brothers in Avenger Penguins would give their pondering reflections on the story.
The whole purpose of the character Kiwi in Chowder.
Parodied in "Spelling Bee My Baby", where after the plot's Star-Crossed Lovers dilemma starts, Principal Lewis appears in medieval garb and recites the opening lines to Romeo and Juliet before shouting "Shakespeare, bitches!"
Sportscasters sometimes adopt this approach, alternately involving themselves with the event through sideline interviews, or standing aside to comment on the action and build up viewers' excitement.