Spontaneous Crowd Formation

The hero and/or villain are about to face off. There's been a massive explosion. Something dramatic is going on. Suddenly, a crowd shows up out of nowhere to watch. Sometimes they serve to keep others from running, in a sort of human Ring of Fire, but more often they've simply Come to Gawk.

For some reason, even if the thing they're watching is dangerous, like a Mexican Standoff, they usually don't think of running unless it directly affects them. When the interesting event is over they go about their business like nothing happened.

They are often Apathetic Citizens, but they don't have to be. Sometimes the crowd provides assistance to the heroes or villains; however, such assistance is usually minor and may be only moral support. Compare and contrast Angry Mob, where the crowd that forms is out for blood with Torches and Pitchforks.

The inverse of this is when a building or hallway full of people clear the area with remarkable speed, panic and thoroughness, so that the hero and the villain can duke it out in an empty hallway, possibly with loose papers flying around. That reaction can often be a form of Crowd Panic.

Comic Books

 * This tends to happen a lot in the Marvel Universe, particularly to Spider Man. Usually the crowd being there will result in the villain threatening them so he/she can get away. At other times, they witness the wrong thing at the wrong time and blame the heroes for whatever happened.
 * Happens a lot in the DC Universe, too.

Film

 * The Spider Man movies had this happen. In the first film, they help Spidey by throwing insults and rocks at the Green Goblin. The second film plays on this after Spidey is knocked out saving the train and passengers; the crowd puts his mask back on him and stands up to Doctor Octopus, only for him to shove them aside, snatch the defeated Spidey, and carry him off.
 * Streets of Fire had the villain summon about 200 bikers out of nowhere. A bit later, a bunch of citizens with guns showed up to stare them down. They then proceeded to do nothing until the hero beat the villain in a street fight, at which point the guys with guns chased off the bikers.
 * Earlier, when Ellen Aim was returned safely, a bunch of people crowded the police station.
 * This is a staple of karate movies, even parodies of them. The Kentucky Fried Movie's parody 'A Fistful of Yen' had this occur when the hero and villain were facing off, with the crowd made up of other karate students.
 * In Star Wars: A New Hope, when Darth Vader and Obi-Wan face off, the Stormtroopers who were guarding the heroes' ship rush over to assist, leaving the ship unguarded. This would have allowed the heroes to escape quietly if Luke hadn't screamed and alerted the troops to their presence.
 * In The Film of the Book of V for Vendetta, after that cop shoots the little girl, a crowd gathers around, looking for blood.
 * In the Tim Burton Batman movie, the Joker announces he will be giving away free money at a parade. Despite knowing he's a murderer, the crowd shows up on schedule and he almost kills them.

Live Action TV

 * The Ray Bradbury short story (upon which the TV episode was based) was "The Crowd". The episode can be viewed here: part 1, part 2 and part 3.

Literature

 * This is an established rule in Discworld's Ankh-Morpork -- no matter what the citizenry are doing, if something interesting is going on, they WILL stop to watch it. Can work for or against people.
 * This is often called the official pastime of Ankh-Morpork.
 * In the Larry Niven short story "Flash Crowd", this sort of thing is one of the unexpected side-effects of the development of easy teleportation technology.
 * As mentioned above, Ray Bradbury wrote a short story entitled "The Crowd" that examined this phenomenon.

Music

 * The Protomen have crowds forming a lot in their songs. Dr. Light Lampshades it a couple times in Act II.
 * They tended to be an Angry Mob when not being Apathetic Citizens. In Act II, they attempted to lynch Dr. Light, believing him to have killed Emily. They gathered around Joe and Light after the former's death, then ran away from Wily's army of robots.
 * In Act I, after idly watching Protoman fight for them, they did nothing as he died. Later, a crowd formed around Megaman as he resolved to fight for the people. But when Protoman was revealed to be alive, and he called them out for doing nothing, they ordered Megaman to kill Protoman. After Megaman left, they were then slaughtered by robots.
 * The Megas have a crowd watching Megaman and Quickman's Showdown At High Noon in the song "The Quick and the Blue".

Tabletop RPG

 * Dungeons and Dragons adventure WG 5 Mordenkainen's Fantastic Adventure. When the PCs fight the iron golem, the viewing stands will be filled with an illusion of a crowd. Some of them will cheer the PCs and some will encourage the golem.

Video Games

 * In the second Paper Mario game, when you enter a battle the scene with switch to a theatre with a cheering crowd. The size of the crowd increases as the game continues, and sometimes they can provide you with items or a power-up.
 * Most Open World Games do this, starting with Grand Theft Auto III. Black and White had people gathering to cheer fights between giant monsters.

Webcomics

 * Invoked in Girl Genius: when Gil needs a crowd, there isn't one, so he purposely picks a fight with Zeetha to attract one.

Western Animation

 * This happened a couple times in Trollz, as a crowd gathered to watch a Mexican Standoff. However, some of the spells hit the crowd.
 * In Spongebob SquarePants, SpongeBob and Patrick start arguing in the halls of Mrs. Puff's driving school and instantly a crowd gathers chanting "Fight! Fight!" It takes Patrick a while to catch on to the fact that they are the ones who are supposed to be fighting.

Real Life

 * A Flashmob is an inversion, as the crowd causes the interesting thing to happen.
 * This is prevalent in real life, especially at the scene of a crime or accident.