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.

Examples of Chekhov's Army include:

Anime and Manga

Fullmetal Alchemist: With an actual army! The Briggs soldiers help out Mustang during the central riot/coup d'etat.
- 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 (who turns out to be a half demon) or an amnesiac princess. 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 Shizuru 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. Until episode 44, revealing that, because they were rescued by the Precures, their hearts were strong enough to withstand Big Bad Dune's assault on Earth and allowing the girls to save the day!

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 almost every single alien species that appears in the first part of the movie returns to beat the ever-loving crap out of the human military. Chekhov's Zoo, anyone?
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 takes Amelie's father's gnome to various places over the world. A seemingly unimportant blind man in the subway then returns later when Amelie helps him cross the street, and then she extravagantly describes all of of the major sights along the route back to the subway.
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 were all revealed to be a part of the Game as shown in the cafeteria scene.
The first Mission: Impossible movie, where every random extra in the background in the opening scene shows up as a secret agent in the last 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? The army conveniently appears at the end of the film, when Arthur needs them to attack an enemy castle.

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, Regulus Black, Kreacher, 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, the entire graduating class, who'd been getting saved by Buffy for the last three years, bands together to form a literal army to fight off the Big Bad. 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:
- the seires 4 finale. Hello again, Martha, Torchwood, Rose, Mickey, Jackie, Harriet Jones, Sarah Jane, the Shadow Proclamation, the missing bees and the stolen planets!
- A Good Man Goes to War: pretty much every other faction the 11th Doctor's seen come back to help, plus a few that only Ten's seen.
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 when Alan's forced to write a story about the town that becomes reality, these are the only people he knows in the town.
The second Assassin's Creed features this, where after The Reveal, the Hooker, thief, uncle, and various other minor characters end up being assassins instrumental in the endgame.
The Mass Effect series, depending on your choices, is apparently building up to this. Spare the rachni queen in the first game? They send a message promising to help you when you call. Upload a virus into the geth's data networks to turn the small percentage that aid the Reapers back to the majority, which opposes the Reapers? This will probably result in the entire geth race fighting the Reapers with you. Chose not to kill Wrex in the first game and perhaps convinced Mordin to make use of his student's data of a Genophage cure? This may also mean that the Krogans will fight alongside Shepard. Saving Tali from exile without getting her father implicated and flat out begging the quarians not to kill themselves fighting the geth will probably get them with you as well. With friends like these, who needs the Council?
- Wait til Mass Effect 3 comes out, until then it's speculation. So far, we know that one of the possible enemy types are rachni husks.
The Aggron, Arbok, Magcargo, Magmortar, Mismagius, and Rhyperior 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, when Mary is about to throw Anne into the gears of the clock tower, the birds fly over and attack Mary, causing her to back up too much and fall to her death
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 the little village up the coast.

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 the day of black sun. Characters from various old episodes and ones we'd forgotten about. It was epic. Which makes it so stunning that they lost.
- 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.