Ghost Extras

These are extras that hardly ever interact with the main characters, even though logically they ought to do just that.

For instance, a Cop Show might focus on six cops who are often seen doing their jobs at a police station. Lots of other cops and staffers walk around, carrying papers or whatever, but none of these other people actually talks to or interacts with the main characters. Nor do they talk to each other, at least not in a way that the audience can really see. They're just filling out the background, making the police station seem more realistic.

There are practical reasons for this trope. If the extras start talking to the main characters, then the extras themselves will become characters. This will require higher wages and proper crediting. Besides that, the writers might not want to bother keeping track of so many extra names and faces, or they might not want to make the audience bother.

Sometimes a Ghost Extra will get promoted and become a proper character. This is a nice trick for the writers, because they don't have to explain where the new character came from; they can just say she was "there all along" but nobody previously talked to her on camera.

Often paired with The Main Characters Do Everything, if the main characters are part of some larger group.

See also: Lower Deck Episode, Faceless Masses and Living Prop. Contrast Unknown Character.

Examples of Ghost Extras include:

Advertising

A Taco Bell ad shows three characters having a discussion about the new "Queso Crunch Wrap" in the middle of an elevator... while the rest of the occupants ignore the three and move their lips to each other without any noise actually being made by them.

Comic Books

In New X-Men Vol. 2, Academy X and New X-Men, there are a number of 'extra' students seen walking in the halls that never speak with the cast or take any notice of any of the things going on in the school. They don't even appear on the squads every students was theoretically part of. A notable example was a pink blob-person who is shown having such a highly active social life in the background. Why didn't the main cast talk to him? Everyone else was. He even had visitors on parent's day when a major part of the plot then was that most of the cast didn't.

Live Action TV

A show where this makes sense due to Fridge Brilliance is Buffy the Vampire Slayer. It's just the typical Weirdness Censor every resident of Sunnydale has.
The Bill was a particularly conspicuous offender. The cast of regular characters numbered around 20 or so, but beyond those we also seen hundreds of others who would always be milling about in the background, etc. The area where this trope comes into full effect it that sometimes they literally did come across as being ghost extras, where we would see the regulars walk around them in corridor shots almost as if they aren't even there at all. Sometimes a "ghost extra" would hold open a door for a regular character, but you'd almost never see the regular thank them...
CSI (various). It is almost as if the main characters and the extras are in completely separate worlds. Sometimes the main characters have dramatic arguments and yell when they make a sudden discovery in the lab, but all the extras just ignore it completely, not even giving them so much as a funny look. Furthermore, all cases require the expertise of people from the main cast and only people from the main cast.
Stargate Atlantis has an episode where Shepherd is told, loudly and over the radio so that everyone can hear it, that something important and dramatic has happened in the control room. He immediately jumps up and runs out of the room while all the extras just stand there.
Dexter features a large police station, but every case ends up being investigated by members of the regular cast - Doakes gets magically paired off with Batista when La Guerta isn't there to be his partner, even though there are presumably many more police officers available.
NCIS: Has this trope bad. None of the other agents seem to do anything but die. Lampshaded in a parody fanfic.

"The man was dragged off by a member of another team, who Tony suddenly realized he had never seen before."

Played with by Lost: while the show has kept a relatively stable slate of extras, and given many of them names and brief interactions, they never actually get to do anything (except help out in day to day survival in the background and get killed.) Even when this is lampshaded, it's done with new actors who've never been seen before, as with Arzt, Nikki, and Paolo.
Occasionally subverted on Battlestar Galactica, as when Gaeta gets angry and the entire bridge stares at him. In the Razor DVD commentary, Ron Moore talks about the difficulties they had with union rules for the scene where all the cast and extras chant "So say we all!" During the build up to The Mutiny they made a point of showing the extras reacting to conversations between the main cast, which were also shown to be intended by one of the participants to be heard by those extras.
Common in Star Trek: The Next Generation, where Enterprise crewmembers are routinely mute while doing their duty. Just think of every time a scene begins with a main character giving a Red Shirt an order, and said redshirt just slinking away silently, or whenever a main character comes to replace a redshirt who was filling in for him or her on the bridge.
And then of course, Star Trek: Voyager which has a small Federation ship stuck at the far end of the galaxy (and even then, they find five more). The producers do deserve credit for having consistent background folks throughout all seven seasons. Though like Lost, if an Ensign starts having actual speaking lines, you might want to look for something to duck behind - many redshirts do get to speak lines and kick ass and then melt back into the background relatively unscathed like a future version of Batman.
Played with in Degrassi Junior High: Cast members would be played by "The PWT Repertory Company", with only the speaking members being credited in the episode. The idea was that each kid would get A Day in the Limelight, with a Story Arc playing in the background and a comedy subplot featuring Arthur and Yick. Probably didn't quite pan out the way the creators intended, but they made a rather game attempt.
Babylon 5 definitely has a good few of these. However, they also subvert/avert it, by making sure to give the extras more to do, and having at least one extra Opt Out at the Line in the Sand. Generally, the extras also do at least react to stuff, including the rather nice scene where Lyta demonstrates her psychic powers.
Taxi had a full complement of drivers beyond the half-dozen-or-so actual characters. Once in a very great while, you might see one of them respond to a cab assignment from Louie, but other than that...
In Frasier, the main characters frequently sit in the same coffee house ("Café Nervosa"), and Frasier and Niles especially seem to be visiting it almost daily for years, yet they never seem to be familiar with any of the other customers. Whenever a minor character is introduced by having a main character meet him/her at Nervosa, it's always someone they've never seen before. Apparently Frasier, Niles, Martin, Roz, and Daphne are the only regular customers Nervosa has. This is especially odd considering that Martin's regular pub, "McGinty's", is depicted as a place where everybody knows your name. Admittedly there's a difference between a pub and a coffee house, but still.
- Similarly, KACL, the radio station which employs Frasier and Roz, is shown to have plenty of hosts and other staff, but most of the time Frasier and Roz only interact with a handful of them.
On Boy Meets World, the other students in the main character's classes often seemed to be in a whole different place altogether. They never really interacted with Cory, Shawn or Topanga or reacted much when one of those three made a scene in class, and the teachers (Mr. Feeny or Mr. Turner) would only call on Cory, Shawn or Topanga in class and were never seen talking with the other students. The latter point was lampshaded in the series finale:

Mr. Feeny: I regard all my students equally.
Shawn: You know we're your favorites.
Cory: Come on Feeny, you haven't even talked to another student for seven years.

Friends, most of the time. This was lampshaded occasionally.
In Life On Mars and Ashes to Ashes both homicide rooms were populated by ghost extras (quite ironic in retrospect), as well as the main cast. At one point, Sam Tyler, who believes he's dreaming, suggests he's going to keep on walking until he runs out of faces.

Western Animation

Lampshaded at the start of the Garfield and Friends cartoon "Safe at Home" in a case of Animated Actors/No Fourth Wall.

Video Games

The crew of the Normandy in Mass Effect is quite consistently there, but not interactive in any way. Their main purpose seems to be being in the line of fire, especially in Mass Effect 2. When they're all taken by the Collectors in the second game, EDI fills in for the entire crew without any problems.
- Note that the only reason she wasn't filling their roles already was the prejudice against A.I.s.

Literature

Phule's Company features a heavily armed force of 200... barely a dozen of whom have names.