In H. G. Wells's The War of the Worlds, an unnamed artillery gunner appears in the main character's house and informs him of the Martian fighting machines, They go to a town together which is later attacked by the Martians, and he disappears in the chaos. Later he re-appears, speaking about how they can build a whole world underground. Over the course of the chapter, this becomes pivotal to the unnamed main character's decision to kill himself by running up to the Martian fighting machine— only to discover it dead, leading to the final chapter of the book.
Early on in A Tree Grows in Brooklyn, Sissy chats with a nameless policeman who tells her his wife is an invalid. A few years later, her sister Katie meets a policeman named McShane, whose wife has tuberculosis. He keeps track of her, and they marry several years later after both their spouses have died.
C. S. Lewis in The Silver Chair has a particularly devious one. Jill and Eustace are sent to look for the kidnapped King Caspian's son. Halfway through their journey to the place where the prince disappeared, they find a delightful young damsel escorted by a silent knight who doesn't show his face. If you haven't read the book, you have correctly guessed by now that the knight is the prince that they were looking for. However, the damsel is, in fact, the Big Bad that appears to the children to point the direction of a castle inhabited by giants for whom humans are refined cuisine delights. And the children never even suspect about the identity of the two strangers until the climax of the book.
In a party scene in Arrows of the Queen, the first of the Heralds of Valdemar novels, a throwaway line mentions Queen Selenay sitting next to a Herald with streaks of white in his hair at either temple. This turns out to be Herald Eldan, who has a significant role in By the Sword.
Similarly, both Herald Lavan Firestarter (Brightly Burning) and Herald Vanyel (The Last Herald-Mage) are mentioned as historical personages long before Mercedes Lackey wrote a book/trilogy about them.
From the Deryni novels: In what amounts to a cameo role, Bishop Henry Istelyn first appears as a previously-unnamed itinerant bishop who delivers a notice of excommunication to Kelson early in High Deryni. In the sequel The Bishop's Heir (set two years later), the loyal Istelyn is elected to the episcopal See of Meara and his fate becomes a major part of the book's plot.
Also done in books two and three of Cinda Williams Chima's High Fantasy smash hit, The Seven Realms Series. In book two, Han convinces Action Girl Catarina to study at the temple school at Oden's Ford, an area of the school known for producing the most refined maidservants in all of the Seven Realms. In book three, Han needs someone he can trust near Raisa, but someone that can also defend her. Turns out those skills Cat got came in handy.
To elaborate, his power is the ability to change his mass, from able to float to sinking through the ground. The sinking through the ground part turns out to be the only way to permanently destroy the gaiaphage, most likely digging it to the center of the earth, assuming his skeleton retains its mass after he dies.
In When You Reach Me, the laughing man seems to have no purpose in the book, but in the end, we learn that he's one of the main character's friends who has traveled back in time to save one of her other friend's lives
Lampshaded in the first Artemis Fowl book with the introduction of Chix Verbil, whom the narration says will become important in sequels, but "for now his only function is to press a button to activate the time-stop".
Played straight with Turnball Root, appeared in a short story and reappeared in the Atlantis Complex.
That sprite who told Turnball how to re-acquire some magic? That's the first supernatural being who ever appeared on-screen: the fairy who loaned Artemis a copy of her Book way at the beginning of the series. Given that she hadn't appeared since the first chapter of the first book, and this was the seventh, this probably is also a Brick Joke.
Something of a bizarre usage in Dan Abnett's Warhammer 40,000 novels, where his early Gaunt's Ghosts stories namedrop characters who he would later develop in future novels, but his Word of God reveals that he hadn't actually planned so far ahead. For example, comments made regarding Inquisitor Gideon Ravenor, who would eventually get his own novels, from the Gaunt's Ghosts Omnibus The Founding:
"... first mention of Ravenor (who could have guessed where that would lead to)...."
As a series with a fifty-four book run, not to mention prequels and Megamorphs books, Animorphs had quite a few of these. There weren't as many as you'd expect, due to the latter half of the series being ghostwritten, but there were still plenty of meaty examples for discerning reading to enjoy.
Visser Four, first mentioned in an aside in The Decision, becomes the main villain of Megamorphs #03: Elfangor's Secret.
In The Arrival one of the four Andalites is presented as a mysterious nobody. He's eventually revealed to be The Man Behind the Man.
The ultimate Chekhov's Gunman for Animorphs, though, has got to be Crayak. The Big Red Eye first appeared to Jake as a vision at the end of The Capture. For the next twenty books this goes unmentioned, and then along comes The Attack and the big reveal.
In the beginning of The Departure, it's mentioned that a leopard escaped from someone who keeps wild animals. The leopard ends up playing a huge role in the story.
The Harry Potter books did this to death, along with the other Chekhov's tropes. There were characters introduced in every book who became plot critical later, both in each individual book and across the series. There were even numerous instances of a character being revealed to be important more than once.
Sirius Black, mentioned in passing in the first chapter of the first book and revealed two years later to be the title character of the third book.
Also, if you pay attention, the Lovegood family gets mentioned early in book four, while Luna didn't become an important character until book five.
Mundungus Fletcher has been getting small, one-time mentions as early as Chamber of Secrets, only becoming even slightly important to the plot in Order of the Phoenix.
Mrs. Figg is introduced in the first book as Harry's babysitter, and we hear very little of her afterward. She is actually mentioned off-hand in book four as part of the 'old team' but it's very easy to forget about her. In book five, she's revealed to be a Squib that's been watching over Harry on Dumbledore's orders for the last fourteen years and is the sole witness to Harry's Hearing at the Ministry of Magic.
Aberforth Dumbledore first appears as the barman of the Hog's Head Tavern in Order of the Phoenix, although he isn't mentioned by name. Earlier, Dumbledore does mention him by name in Goblet of Fire.
The film makers wanted to leave Kreacher out of Order of the Phoenix entirely, but JKR told them that he would become very important later, as revealed in Deathly Hallows. (The Deathly Hallows film, however, cuts so much of Kreacher's role that his omission wouldn't have created any issues.)
Even Neville Longbottom could be considered one of these; even in book one, he has little to do with the overall plot, and he remains a Bit Character until Order of the Phoenix. In Deathly Hallows he cuts the head off of Nagini, the final Horcrux. He probably could have been introduced later like Luna was and it wouldn't have made much difference to the plot.
The savvyness of Potter fans in general meant many were delighted to realize, on rereading, that "that awful boy" Petunia refers to in Phoenix who taught Lily about Dementors is not James, as is assumed by Harry and the reader, but Snape.
Another living Chekhov's Gun comes in the first chapter of Goblet of Fire, which seems like an unnecessary bit of exposition regarding Voldemort's ultimately-unimportant Muggle father and the fact that Wormtail has found Voldemort, in which the two of them kill a Muggle. However, this is also the first appearance of Voldemort's snake Nagini, who later takes on a more important role. Each of the next three books makes this chapter more clear. In OotP, Voldemort sends Nagini to attack Arthur Weasley — and Harry, who has been having visions in which he sees what Voldemort sees, is somehow able to see the scene from Nagini's perspective. The chapter itself is explained in HBP, when we learn that a murder is required to create a Horcrux — what we were seeing was Voldemort turning Nagini into a Horcrux, using the murder of the Muggle as the impetus. Finally, in DH, we learn that Harry is also a Horcrux, which is why he was able to see through the eyes of both Voldemort and Nagini.
Scabbers is another prime example, making the rather unorthodox transition from Sidekick's Pet to Henchman in the third book.
Griphook, who has a total of two appearances throughout the series, six books apart - the first as a classic Underground Bank Cart Pilot, the second as a Temporary Ally In Bank Robbery.
Bellatrix Lestrange is on trial with Crouch Jr. in the Pensieve Flashback in Goblet of Fire. By the end of the series she's the most prominent and loyal Death Eater.
Grindelwald, who was mentioned briefly in the first book in Dumbledore's Chocolate Frog Card description as an evil wizard whom Dumbledore fought. The last book gives him a lot of backstory, including the revelation the he was once Dumbledore's friend.
Both Broderick Bode and Sturgis Podmore were offhandedly mentioned throughout OotP, and were thought to be the most insignificant of characters until Hermione brilliantly pieced it all together and figured out what had happened. (Bode even had a blink-and-you'll-miss-it introduction in the previous book.)
Rufus Scrimgeour is offhandedly mentioned by Order of the Phoenix members in the fifth book before taking over as Minister for Magic in the sixth.
The wandmaker Gregorovitch was mentioned by Ollivander in the fourth book, and serves a brief but important role in the seventh as one of the former owners of the Elder Wand.
Though fan attentiveness paid off in Half-Blood Prince when Harry gets the fake locket with a note by R.A.B. Fans were attentive enough to guess that R.A.B. was Regulus Black, the brother of Sirius briefly mentioned in Order of the Phoenix - mentioned all of twice in the series up to that point; we're talking some very attentive fans here - based on the fact that Regulus was mentioned to have run off to be a Death Eater, then was seemingly killed by Voldemort for getting cold feet, which would have made him a good fit though fans weren't told Regulus's middle name. In fact, fans sussed this with so little trouble, some fans argued against it because it was too obvious. Rowling's response to the theory was "that's a really good guess."
In the Legends trilogy of the Guardians of Ga'Hoole series (books 9-11), a character named Theo is introduced midway through book 9, and he becomes the first blacksmith and creates the first battleclaws, but that's about it. However, later on, in book 13, it's revealed that Theo wrote the holy scriptures of the Middle Kingdom, and as such is directly responsible for the pacifist lifestyle of the owls of the Middle Kingdom, as well as being indirectly responsible for all the events involving the Striga, an escaped Dragon Owl who allies himself with Nyra and the Pure Ones in book 15.
In Sandy Mitchell's first Ciaphas Cain short story, we are introduced to Cain's rather smelly and loyal aide Jurgen, who doesn't look like he'll have much relevance except for jokes on how he puts people off with his atrocious hygiene standards. Later on, it turns out Jurgen is an extremely potent "blank," someone who negates psychic powers and harms daemons simply by being in proximity to them, and his becomes a constant and critical plot point throughout the rest of the novels. His absolute loyalty and obedience to Cain also play important roles.
The Dresden Files is very prone to this - the main opponent of a book will almost definitely be someone who was introduced in an earlier book, quite possibly Grave Peril.
Turn Coat has another major one: The obnoxious little secretary wizard who tries to get Harry to sign for a folder he was getting off the record turns out to be (one of) the traitor(s) on the Council. And he was actually trying to get Harry to sign because he was using special ink for signatures to screw with the wizards' minds.
Said character was actually introduced as far back as Summer Knight in passing.
Along side this character was a woman who was described as old and holding a scepter. It is later revealed to be Anastasia Luccio, who is Captain of the Wardens, eventual lover of Harry Dresden, and brainwashed victim ordered to kill.
Molly Carpenter. Initially just one of Michael's many kids, she is first mentioned in passing in Grave Peril by Lea, Harry's fairygodmother briefly appeared in Death Masks as a background character. She becomes a major player in later books when she begins to manifest magical abilities and Harry takes her as an apprentice. At the end of Cold Days, she becomes the new Winter Lady.
Mab, Queen of Air and Darkness, gets name dropped by Harry as far back as the first book Storm Front. She would end up becoming a major player in really short time.
Butcher also does this a bit with Codex Alera — only the gunman turns out to be a "gunspecies". Tavi and Kitai go into the Wax Forest for a test and end up not only encountering the wax-spiders, but also an unusual new creature The Vord queen. They're mentioned in passing during the first book — but show up in each new one, getting more dangerous each time.
American Gods: Shadow's old cellmate, Low Key Lyesmith. Keep in mind that Norse gods play a huge role in the book, and he is actually Loki Lie-smith.
In Tad Williams's Otherland series, the side plot involving Olga Pirovsky is treated with a great deal of significance even though it's not initially apparent how her mysterious headaches have anything to do with the main story. Even when she's tasked by Sellars to infiltrate the headquarters of J Corp, it seems like her role is fairly straightforward. Then comes The Reveal, and she turns out to get the biggest Crowning Moment of Heartwarming in the whole series.
Ezra Jennings from Wilkie Collins's The Moonstone is the assistant to Dr. Candy, himself a side character. Both of them, but especially Jennings, are more important to the plot than the reader might first think.
Emile Zola loved to use this trope in his Les Rougon-Macquart series. A character mentioned in passing in book one and described by his father as a forgettable good-for-nothing shows up as the main character in books 10 and 11. Another one mentioned in passing in book 3 is the main character of book 14. The pattern repeats itself throughout the books. Things get even more confusing when you find out that the books do not follow in chronological order and that the timelines of most of them intersect in one way or the other. Trying to keep up with who is doing what and is important in which book can become a nightmare.
Not to mention Boromir's passing references to his father, the Steward of Gondor, who becomes a major character in The Return of the King.
Gandalf mentions Aragorn and the Nazgűl in the second chapter. Gandalf even points out the fact that he mentioned the Nazgűl to Frodo before any of the characters even met them.
The Necromancer is given a passing mention in The Hobbit. It is only in the sequel that he becomes important since he is actually the reborn Sauron.
In Silas Marner, the character Godfrey Cass is seemingly of no direct importance to the main plotline, until his brother robs Silas, and then his secret daughter ends up in Silas's care. Small world!
InThe Wheel of Time every named character in the book will return with greater importance latter on. Every. Last. One. It's even justified by the fact that the three main characters bend chance, circumstance, and the fabric of the universe itself simply by existing.
Atlas Shrugged: Who is John Galt? The unnamed minor character standing in plain sight for six hundred pages, rather than a rhetorical question.
A head of one of the smuggling organizations with a subplot in The Thrawn Trilogy has bodyguard, and in one line he calls her Shada. She's good at her job. Zahn later wrote a short story called "Hammertong", in which she was an all-around agent who was one of the many people who helped get the Death Star plans to Leia. In the Hand of Thrawn duology written five years after The Thrawn Trilogy, Shada Du'kal accompanies Talon Karrde on his search for a certain document, and has her own Mystral subplot.
In J.C. Hutchins'a 7th Son trilogy, characters like Special K, Peppermint Patty and Klaus Bregner get throwaway mentions only to become very important later on.
When first introduced in Warrior CatsInto The Wild, Littlecloud is just a small, seemingly unimportant apprentice from ShadowClan, but later becomes the ShadowClan Medicine Cat after Runningnose and a good friend to Cinderpelt.
In The Silmarillion, Lúthien Tinúviel is introduced off-hand by Tolkien simply as the daughter of Thingol and Melian, but later, as we all know, plays a major part in retrieving a Silmaril by owning Sauron's face and tricking Morgoth himself and proving herself to be totally Badass.
In Red Storm Rising, the Soviet Union, suddenly faced with a crippling oil shortage, decides to conquer the Middle East for oil. To do this, they first need to eliminate the threat NATO posed to the operation. Their plan was to detonate a bomb within the Kremlin, killing several staff members and 8 children from the city of Pskov, then blame it on West Germany and invade, hoping that the other western nations would object to being bled white to defend what they would see as a terrorist regime. The funeral is described in great detail, and the viewpoint character of the segment, a non-voting politburo member named Sergetov, focuses on a grief stricken captain of paratroopers, whose daughter's body was so mutilated that her face was draped in black silk for the open-casket ceremony. Near the end of the book, the chairman of the KGB and Sergetov join up with the most senior surviving Soviet general (most of the rest had been shot for failure) in a coup to prevent the deployment of nuclear arms at the battlefront. After taking power, the general turns to the KGB man and the following conversation takes place (paraphrased):
General: By the way, Comrade Lidov, have you met my new aide? He had a daughter in the Young Oktoberists. KGB Chief: Your point? General: His regiment is based in Pskov. Aide: For my little Svetlana, who died without a face. (fires)
Matthew Reilly tends to introduce these early in the book when they become useful.
Ice Station has Trever Barnaby, Jack Wash and Chuck Koslowski mentioned in Schofield's thoughts about his mission. O. Niemeyer also turns up early in an investigation about the events of the book and is currently MIA, but turns out to be a subversion because he died in a plane crash.
Temple has Will's brother Martin Race, supposed to be working with the team remotely.
Six Sacred Stones introduces Jack West's brother in law and previous neighbour in a flashback.
In the Young Wizards books, there are a decent number. Two of note are Ponch, who in an incidence of Chekhov's Gun does indeed get stranger the more time he spends with Kit, as wizards pets do. He crosses into and even creates alternate realities and is implied to maybe be God. Who apparently likes palindromes. The other is Biddy in A Wizard Abroad who is the local smith and farrier and turns out to be one of the Powers That Be and helps reforge a powerful artifact that has been lost.
Romance of the Three Kingdoms inconspicuously introduces Sima Yi as Cao Cao's secretary, and you might not expect much from him in a novel with Loads and Loads of Characters (especially considering how the founders of the eponymous Three Kingdoms are introduced). However, Sima Yi eventually becomes Zhuge Liang's rival, before staging a coup during the reign of Cao's great-grandson. His grandson becomes emperor.
A particularly impressive one from David Eddings. In The Ruby Knight, the second book of his Elenium trilogy, Zalasta is introduced in what is practically a throwaway scene; literally, he's seen for less than a page. He's not seen or mentioned again in the entire trilogy... and then he shows up in the Tamuli trilogy as one of the major characters, and ultimately turns out to be the closest thing to a Big Bad for both trilogies.
In Elizabeth Honey's Remote Man, the book's antagonist is introduced in Chapter 5 as an unpleasant American tourist who buys seven paintings from Ray's gallery in Arnhem Land. A few chapters later Ray's daughter Kate comes to suspect that he was responsible for stealing a rare python, based on the fact that her cousin Ned unwittingly told him where to find it. About halfway through the book, Ned learns that he's a retired Hollywood stunt driver running an international smuggling operation.
In The Chronicles of Ancient Darkness, The Walker, a crazy old man, is introduced in the 1st book, but it isn't till the sixth that we learn he is the 7th Soul Eater. He did give us some hints, though, like when he said that before he went insane, he was "a very wise man".
In The Caves of Steel, R. Sammy, a robotic courier, is the first "person" we meet in the book, but he seems more of a simple narrative device to provide an opportunity for exposition about the Earthmen's dislike of robots. Much later on, he is found destroyed, and it turns out he was a critical part of the book's murder plot - the one who sneaked the murder weapon into the area before the crime.
Sarah Caudwell's Hilary Tamar mysteries have a tendency to mention elderly relatives of the protagonists in passing who will become crucial to later books in the series. The first book, Thus Was Adonis Murdered, mentions that Cantrip has an uncle who's a Second World War veteran; he ends up providing some crucial exposition and a Big Damn Heroes moment in the third book The Sirens Sang of Murder. The Sirens Sang of Murder in turn mentions Julia's Aunt Regina, who narrates a sizable fraction of the fourth book, The Sibyl in Her Grave.
A few of the main characters in The Pale King go unnamed in their introductory chapters.
In Death series: At one point in Loyalty in Death, an old man mentions how he has met with two guys with dead eyes who are just the muscle and working for terrorist organization Cassandra. The two guys become the murderers to hunt down in the Survivor in Death story. In Judgment In Death, a blonde-haired assassin working for Max Ricker kills off a mook who was going to reveal a lot of details. The blonde-haired assassin turns out to be Max Ricker's daughter and she becomes the murderer to hunt down in the Promises in Death story.
The first Dinotopia book featured a minor character named Lee Crabb. He ends up becoming the antagonist in all the subsequent books (barring First Flight, which is a prequel).
In Craig Shaw Gardner's "Cineverse Cycle," several minor characters from the first and second books were later revealed to be disguises used by a superhero called Captain Crusader, whose decoder rings could transport the wearer into and through the different worlds of the Cineverse whenever they said the code phrase.
In Lunar Chronicles by Marissa Meyer, it is mentioned that Princess Selene of the moon had died in a fire- but that no body had been found, just some pieces of burnt flesh. Turns out that Cinder- the cyborg protagonist- was really Princess Selene, who had been rescued successfully and had been given artificial replacement parts!
In The Long Earth, Rod Green is first introduced as a son of a family which intends to colonize a parallel Earth. Unfortunately, he cannot "step" to parallel Earths, so his family abandons him. He shows up a couple more times where it becomes apparent that he resents his family for his abandonment, but only features prominently when at the end of the novel, where he blows Madison, Wisconsin up with a nuclear bomb.
Trapped on Draconica: Subverted. Daniar busts a drug runner early on in the narrative and he shows up later working for the Big Bad and out for revenge. Daniar busts him again without any trouble then or later.
Griffin's Daughter: Lady Amara mentions her predecessor as head of the elf mage circle had disappeared years ago. A book later, Amara's son - after being sold into slavery by humans - runs into Gran, an older elf maiden who turns out to be the aforementioned missing mage. (she was there of her own free will, as penance for an earlier tragedy.
In the opening of Dorothy Gilman's The Clairvoyant Countess, Madame Karitska does a reading on Lt. Pruden and tells him that he will be married within fifteen months, to a woman with long, very pale blond hair and considerable psychic ability. When, later in the book, he is not able to talk straight upon meeting a woman with long, very pale blond hair and considerable psychic ability, he doesn't get it, but Madame Karitska smiles upon him and assures him that it is a very good thing.
In The Hunger Games, after Katniss shot an arrow at the Gamemakers she said that one of them tripped backwards into a punch bowl. She has a little interaction with him during her pre-Games interview, then he is never mentioned again. Until the next book, Catching Fire, when she formally meets him and finds out that his name is Plutarch Heavensbee and he is now the Head Gamemaker. At the end of Catching Fire, she finds out that he is also part of a Capitol rebel group, and helped orchestrate the plan to break the victor-tributes out of the Quartel Quell arena. He then becomes a major character in Mockingjay and a significant player in the war effort for the rebels.
In Native Son, Bigger sees Buckley's face on a campaign poster early in the story, long before being prosecuted by him for rape and murder.
Prince Oswin notes in the very beginning of Terra Mirum Chronicles that anyone useless is disposed of during a revolution. So why is Erebus still alive after the Nightmare Queen has everything she should need from him?
In the Circle of Magic quartet's first book, Tris mentions to Niko that she has a cousin Aymery at Lightsbridge. In Circleverse, Aymery is as common a name as Steve, so when she hears it on the wind in Tris's Book she dismisses it as coincidence. Aymery himself arrives at the Winding Circle temple to visit a bit later acting as a spy/saboteur for the pirates who want to raid the place.
Most notable is Thomas Theisman, who shows up in Honor of the Queen and has a minor role (well, not that small, but still, he's not even a secondary character). Then he appears briefly in The Short Victorious War and Flag in Exile, presumably due to the Law of Conservation of Detail, and takes a gradually bigger role in the narrative, until he finally overthrows the evil government of his own country, restores the truly democratic one that hadn't existed for two centuries, beats the crap out of the Royal Manticoran Navy, rides to their rescue with his navy after they've had the equivalent of Pearl Harbor pulled on them and then signs a military alliance with them, shares a flag deck with Honor Harrington herself, then does some more awesome stuff... and he probably hasn't finished, as he has ascended to be a series anchor in a World of Badass.
Honor's mum, Allison Benton-Ramirez y Chou Harrington. Mentioned briefly at the beginning of the same book as Theisman, she becomes more and more relevant from In Enemy's Hands on. Not to mention that by A Rising Thunder we find out that her brother is a Very Big Fish in the Beowulfan government. Of course, this branch of the family was mentioned earlier, but still...
Eloise Pritchart. From relatively minor POV character in Honor Among Enemies to President of the Republic of Haven and series anchor in her own right in four books or less!
Those That Wake has Isabel, who appears early on and shows up again in the middle of the book.
A Fantasy Attraction has Aleksandra, a dragon who is collecting for her hoard. She is invited in, and you promptly forget about her. She shows up later just in time to incinerate a tribe of ogres.
In Dangerous Spirits, Niki's father receives a vague reference in passing in Green Fairy. Turns out it's Konstantine.
The Reynard Cycle: In Defender of the Crown, Reynard follows the trail of an assassin to the garden of a herbalist, where he meets with an assistant. It is later revealed that the herbalist is Hermeline, Reynard's scorned ex-lover, who uses the opportunity to sell Reynard poison rather than a sleeping drug. As a result, Reynard ends up accidentally killing the Queen.
A Song of Ice and Fire has egregious numbers of Gunmen. An outstanding example would be Lord Beric Dondarrion, who was dispatched in the first book to deal with outlaws, only to show up two books later. Almost every important character, if not introduced, was at least mentioned a book prior.