Teaser-Only Character

At the beginning of many shows—most commonly in Police Procedural or Monster of the Week shows—a nameless generic character or two will wander around some deserted location, frequently either on a date or up to hijinks, and then one of three things will happen:

a) They will come across a horribly mutilated corpse.

b) One or more of them will be brutally killed.

c) They will narrowly avoid being killed.

This only lasts about a minute, or even a matter of seconds, before we cut to the show's characters, who have come in to investigate the death, deaths or deadly threat. The character/s from the teaser have almost no involvement from this point on; they might be asked one or two questions, but they're just as likely to get nothing but a brief mention ("A couple of kids found her three hours ago", etc) and that'll be it for them.

The overwhelming majority of these characters are good-looking teenagers.

Obviously this doesn't apply to characters who die in some mysterious semi-obscure way in the opening seconds of a show, then go on to be the subject of inquiry, flashbacks and a thorough autopsy.

See also Start to Corpse.

Film - Animation

 * Leland Turbo from Cars 2.

Film - Live Action

 * There was a short-lived fashion in action thrillers to open with a detailed buildup of a character, such as would usually introduce a protagonist, and then just when the reader was getting to like them the bad guys would kill them (thus showing right off how bad they were) and the real story and the real protagonists could come in.

Literature

 * All (or most) of the point-of-view prologue and epilogue characters so far in A Song of Ice and Fire have met somewhat grisly ends. But then, so does almost everybody else.

Live-Action Television

 * Favored by the Law and Order shows, CSI, The X-Files, Supernatural and Buffy the Vampire Slayer. Buffy actually subverted this trope in its first episode; the opening sequence featured a good-looking teenage girl intruding on school property after hours with a date (making it both a date and hijinks). We think she's a generic soon-dead girl, but then she goes on to be a main Recurring Character throughout the history of BtVS and Angel. (Her date, on the other hand, was a pure example of this trope, as the aforementioned girl proceeds to suck his blood.)
 * Neatly parodied in Police Squad!!, where every episode had a guest star who was killed while their name was being announced in the credits.
 * NCIS does this a lot.
 * Steve Urkel from Family Matters used to be one of these.
 * House Has them almost every episode, with the audience spending most of the teasers guessing who will be the patient and who will be this.

Western Animation

 * Justice League episode "Darkheart" opens like this, with a couple of rock climbers scaling a plateau, before finding alien nanobot things. It cuts away immediately. One of them is seen answering General Eiling's questions as the League arrives. As for the other, well; the teaser ends with the aforementioned partner finding their empty boot before the camera reveals the growing nanotech mass, so draw your own conclusions.
 * Amusingly played with in an episode of Justice League which begins with a team of workers on an oil rig in the desert, then there's an explosion... and the frame freezes and zooms out, revealed to be a presentation put on by a Corrupt Corporate Executive.
 * "Eclipsed" opens with a military unit hunting a warlord named Fessan. They get a few minutes of screentime before one of them gets possessed by the Villain of the Week and kills them all.