Chekhov's Army

Chekhov's Army is to Chekhov's Gunman as Chekhov's Armoury is to Chekhov's Gun.

A Chekhov's Gunman is a character introduced in a seemingly small role and brought back into the spotlight later with a much larger, and generally more important part to play.

Chekhov's Army is when the writer uses several (and in some cases, uses too many) Chekhov's Gunmen, not all of which are painfully obvious.

The Law of Conservation of Detail taken to its logical extreme.

Carefully written and/or Myth Arc-laden shows tend to have a Chekhov's Army. It also provides good potting soil for Epileptic Trees.

Compare with Chekhov's Armoury. Might overlap with Gondor Calls for Aid and Battle Royale With Cheese.

Anime and Manga

 * Fullmetal Alchemist: With an actual army! The Briggs soldiers help out Mustang during
 * The 2003 anime version: Every single named character who doesn't die in his first appearance shows up later in the series.
 * In Mahou Sensei Negima, roughly half of class 3A (all of which are presented in the first chapter) eventually turn out to be a Vampire or a Ninja or a Half-demon Samurai or a Time-traveller or a Robot or a Ghost or a Nun/Wizardress or a Mercenary marksman or   All this is without counting all the side characters which inevitably become very important to the plot.
 * My-HiME: Every single HiME that didn't show up in the first episode, Nao and  being the most egregious examples.
 * In Mai-Otome, several heads of state and their Otomes attend Mashiro's coronation. The final battle is between The Alliance of nations trying to liberate Garderobe, and Nagi's minions and allies.
 * Monster is very good at this.
 * So many in One Piece, and some of them take a long time to come into play.
 * Chances are, if you have a name and appear in Pokémon Special, you're going to be plot-important later on. If you happen to be nameless, however, you will still appear some chapters later as either an ally or a throwaway gag.
 * Heartcatch Precure spent its entire series building up its army! Almost every episode was the same - character A suffers from some predicament that draws them into despair, Desert Apostle turns them into a Desertian, Precure comes and revert the person to normal. With an exception of a few people, most of them don't show up again.

Comic Books

 * The Sandman probably has the most extensive one ever. Some notable examples:
 * There is a lesbian character killed via Moral Event Horizon in Preludes And Nocturnes. Her girlfriend is the main character of The Time Of Your Life.
 * In the very first issue, Dream is captured, and a widespread "sleeping sickness" results. One of the victims is the grandmother of the main character of The Doll's House, Rose.
 * Who is herself connected to the two characters mentioned above.

Film

 * In the 2009 movie Avatar, there is a moment where
 * In the French film Amelie, many characters are introduced extensively with seemingly no point. For instance, the air hostess that Amelie helps with pet sitting is the person who . A seemingly unimportant blind man in the subway then returns later when Amelie helps him cross the street, and then she extravagantly.
 * The Game - This 1997 Michael Douglas film fits this trope to a T when every last extra whose face was shown during the course of the movie, as well as several incredibly minor, even unnamed characters
 * The first Mission: Impossible movie, where every random extra in the background in the opening scene
 * Taken literally with Monty Python and the Holy Grail. In the 'extended version', in which 23 originally cut seconds were restored, one of the characters breaks the fourth wall and starts talking about if her scene should have been cut out. This triggers a chat between her and some past characters. After that, some future characters start yelling at her to get on with the scene, including an actual medieval army. Guess what?

Literature

 * The Dresden Files loves this sort of thing; in particular, Grave Peril has quite a few characters show up at Bianca's party who turn out to be very important later.
 * 10 books later and the fallout from that party still isn't done yet. There's one character that hasn't been seen since (a dragon) but the amount of pure speculation means he has to come back at some point.
 * Harry Potter. Sirius Black,  Mrs. Figg, and Cornelius Fudge are just a few examples of characters whose first appearances were rather innocuous, if not downright unimportant, but who would become much more relevant later on. Rowling's penchant for this sort of thing led to her publicly apologizing when a throwaway character had the same last name as Harry's mother; though he really was just a random character, the fandom had grown so used to innocuous characters becoming important, especially those with noteworthy names, that there was major speculation about what role he would play in the future books.
 * The Wheel of Time - Every named character in the series.
 * The Red Army from Terry Pratchett's Interesting Times.

Live Action TV

 * Buffy the Vampire Slayer was fond of this. First, at the end of season three, Two of the three Big Bad Wannabe 's of season six, conversely but similarly, started as one-off characters going as far back as season two, but joined up with each other in season six. The third was going to be as well, but the actor was unavailable so they made it his brother instead.
 * In the first season of Chuck, in Chuck vs. the Alma Mater, the titular character finds a disc of everyone at Stanford who is working for the CIA... and uses it (and a regular Chekhov's Gun in the form of a code phrase about a toga party) to get a lot of reinforcements to bring in the episode's Villain Of The Week.
 * Doctor Who:
 * . Hello again, !
 * A Good Man Goes to War:
 * In The Famous Jett Jackson, on an episode of the Show Within a Show, Silverstone; the main character discovers that nearly everyone he met in the town was actually a secret agent. Except of course for the sweet old lady, who was actually the villain of the episode.
 * Heroes. Oh man, Heroes.

Video Games

 * Absolutely everyone in the first half of the first chapter of Alan Wake plays an important role in the plot later. This actually makes sense, because
 * The second Assassin's Creed features this, where
 * The Mass Effect series, depending on your choices, is apparently building up to this.
 * Wait til Mass Effect 3 comes out, until then it's speculation. So far, we know that one of the possible enemy types are.
 * seen during the Bad Future sequence from Pokemon Mystery Dungeon: Explorers of Time/Darkness/Sky.
 * In Clock Tower, Anne can save a flock of birds early in the game and they fly off. Much much later,
 * Similar to the above, in World of Warcraft, there is one quest chain where the player saves a bunch of baby wyverns who follow the player around for a bit. Later on in the chain, when you confront the person who had them trapped, the wyverns fly in, swarm him, lift him up, and then drop him off a cliff.
 * Heck, it's actually all over the place in World of Warcraft.
 * The Twilight's Hammer. Minor antagonists very early on in Classic, show up much much later in Classic at Silithus, and then become The Organization in Cataclysm.
 * The Cenarion Circle. Depending on where you go, they're a very minor faction who later on is the driving force behind several zones' storylines.
 * The Earthen Ring. Originally, the Earthen Ring but with Shamans. Later on, they become one of the main driving forces for good in Cataclysm, after having several pre-expansion storylines dedicated to them.
 * The Argent Circle. At the low level game in Classic, they appear to be just one of many other factions...and were one of the driving forces for good in Wrath of the Lich King.

Webcomics

 * Looking for Group does this with

Web Original

 * Every single named character who showed up at any time in the Global Guardians PBEM Universe would be back later to play a larger part in the story. Every single one.
 * One exception: The cat burglar.
 * Sketch Comedy The Onion movie had just about everyone from the previous sketches appear in some manner in the climax.
 * Whateley Universe example: all the devisers and gadgeteers from the Whateley Weapons Fair in "Ayla and the Tests". Knick-Knack turns out to be the incarnation of Hephaestus. Kew turns out to be the gadget girl for the Spy Kidz, and gives Phase an intro to said team. Mega-Death turns out to be a good guy who sells Phase the very thing Phase will need when the Weapons Fair 'goes Westworld'. Jericho has invented a new kind of first-aid kit which Phase wants to market. Hazmat turns out to be interested in the protagonist of another series of stories. The list just goes on and on and on...
 * And then most of them come back for a Big Damn Heroes scene in "Ayla and the Great Shoulder Angel Conspiracy" when two devisers put Ayla and Tennyo in the hospital and then try to get away with it.

Western Animation

 * In the mid-season finale of Avatar: The Last Airbender, a lot of characters came back to fight on . Characters from various old episodes and ones we'd forgotten about. It was epic.
 * The Kyoshi warriors are this on more than one occasion.
 * Justice League Unlimited. The Ultimen, the Question, Lex Luthor, Brainiac, Amanda Waller, Huntress, Galatea, and Captain Atom (among others) all have larger roles in the season two myth arc and the four part finale than their initial appearances would suggest.
 * Everyone in The Spectacular Spider-Man. Everyone.