The love interest in Romantic Comedy movies will sometimes be this. Like in Music and Lyrics, Sophie first appears to water Alex's plants before she starts spouting off song lyrics.
Cube includes a seemingly detrimental character, Kazaan, who turns out to be an idiot savant, and critical to solving the formula needed to escape.
Billy Madison provides quite a literal example with Steve Buscemi. When Billy calls him to apologize for being a jerk to him when he was a kid, Buscemi seems as if he'd only be in that one scene as a random oddball for comedy's sake. But just as Sandler is about to be killed Buscemi comes out of nowhere to shoot the would be assassin, saving Billy's life.
Master and Commander: The French captain is not seen by the audience the whole time (The Faceless), until Aubrey finds his body in the sickbay. The ship's doctor could not save him from death, but hands over the captain's sabre to Aubrey. Later, however, Aubrey's companion and friend Dr. Stephen Maturin realizes that the French ship's doctor died months ago, and that the captain therefore must have dressed up as the doctor. The film ends with Aubrey pursuing the French ship once again.
The ace pilot, Wedge Antilles, in the Star Wars Trilogy. Despite not having hardly any screen time, he's shown over time to have a huge impact in helping the rebels win the war.
Throughout the movie Office Space, Milton's storyline is seemingly unconnected to the main plot, as inconvenience after inconvenience roll his way and he occasionally mutters a threat to burn down the building.In the end, he makes good on his threat, destroying the evidence of the main cast's embezzlement (while also stealing the money himself) and setting up for a happy ending.
In The Dark Knight, the camera pays a certain amount of attention to Jim Gordon's fellow police officer Ramirez. Near the end, cue The Reveal that Ramirez is the corrupt cop who drove Rachel to the place where she would die. The viewers who were completely sure she was a Captain Ersatz of Rene Montoya were either outraged or relieved.
A bit of a Double Subversion in fact. At one point in the film, the Joker says he'll blow up a hospital if citizens let a certain man live. Batman asks Gordon to look for cops with relatives in hospitals, thinking they may attack the man. Observant viewers will remember that Ramirez's mother is in the hospital. She does not attack the man, subverting Chekhov's Gunman for the moment.
City of God features this type of character in Knockout Ned's story. The character Otto is seen briefly when Knockout Ned murders his father at the bank. In this case, the gunman is literally a gunman.
In the film The Hunt for Red October, the submarine's cook suspiciously lingering in the background when the political officer's body is bagged is at last revealed to be the saboteur.
Played by the same actor who is finally revealed to be the hitman in The Bodyguard!
Well, Ramius does ask his political officer how many KGB agents are aboard the Red October. The political officer simply says that he'd be the last person to know about any agents. It's surprising there was only one on a top-of-the-line submarine with an experimental system.
Lampshaded in The Amateurs. Two characters are having a discussion about who they want in their porn movie (don't ask), and discuss that one character's brother is out of town, and worry about how tough the brother is and what he would do if he found out. Eventually, the narrator steps in and says, "OK, we know you're not idiots, you've probably guessed by now that the brother is going to be important later."
Wayne's World parodies the Scooby-Doo version by introducing "Old Man Withers" briefly and early; later on, in one of the endings, he turns out to be the villain.
In Training Day, Hoyt breaks up an attempted rape early on and picks up the victim's dropped wallet. Later, when Hoyt is trapped by the Latino gang, they find the wallet on him and it saves his life - the girl he saved was the cousin of the gang leader.
The old military veteran in New Jack City, ends up being the one who finally kills Nino Brown in the end.
In Heat, Donald Breedan (Dennis Haysbert) is introduced in an individual scene getting a job as a short-order cook at a diner. He then does not appear until Neil recruits him to substitute for Trejo as the getaway driver.
In the film Hoffa, the young truck driver having problems getting his assignment, during the present-time cafe scenes, ends up being the assassin who kills Jimmy Hoffa and his best friend.
In Stranger Than Fiction, while the movie is introducing the main character, Harold Crick, it also introduces two seemingly unimportant characters for flavor: a boy that has just been given a new bike, and a woman searching for a new job. Throughout the film they appear randomly as they go about their day-to-day lives with very little fanfare, until the end, when Harold is nearly killed saving the boy from being hit by a bus (driven by the woman on her first day on the job) after he falls off his bike in front of the bus stop. It turns out that the writer of the story Harold was the main character of had been foreshadowing their appearance the entire time.
In Changeling, a young Canadian on a warrant for illegally entering the US is briefly mentioned. He is later pivotal to the plot, when he says he was coerced into killing several children, and identifies Walter Collins as one of his victims..
Why would The Rock be playing a seemingly unimportant, friendly agent in Get Smart? It's not because he's the mole, is it?
In Saving Private Ryan, the squad releases a lone captured Nazi machine-gunner. In the final battle, he is personally responsible for at least two American deaths before he is gunned down by the same American translator who advocated his release.
Charlize Theron is a bit too big of a star to merely play a disapproving housewife in Hancock.
A literal example of this trope reveals the hero of The Good, The Bad and The Weird.
Jigsaw from Saw is almost the poster example of this trope. He gets introduced and shown in the middle of the movie for 5 seconds as a dying cancer patient and poses dead throughout the entire movie in the bathroom, and then at the end of the movie gets up and confirms Adam having lost his game.
Also played with at the end of the movie: Zepp first appears in just a split second scene in which he reminds Gordon of Jigsaw's name. The movie then hints at him being Jigsaw.
There's also Detective Hoffman, who first appears in Saw III for all of thirty seconds, then becomes an important character in IV, the end of which reveals that he's Jigsaw's second apprentice. He then becomes the central antagonist of the remaining films.
But the ultimate example in the series is Dr. Gordon, who is one of the main protagonists of the original film, then disappears until the final film (six movies later), where it's revealed that he was Jigsaw's true apprentice, which he demonstrates by locking Hoffman up and leaving him to die.
In Cemetery Man, Francesco's friend Franco seems a bit character. All that happens between the two is a bunch of phone conversations in which they muse about life and the very slow churn of gossip in the town. However, every time Francesco meets his friend in person, Franco doesn't seem to acknowledge his presence. This is a very important detail.
In Casa De Mi Padre DEA Agent #2 seems like a throw away character until he shoots DEA Agent Parker, who's about to kill the protagonist Armando near the end of the movie.
The two ranchhands also serve as such, as when Armando runs out of bullets on his rifle the two come just in time to bring a second rifle to him so he can kill Onza.
Scarface: Tony Montana first sees Sosa's assassin with the sunglasses, while he tortures and kills Omar for supposedly snitching on the Diaz Brothers. Later, the sunglasses wearing assassin, ends up being the one who finally kills a drugged out Tony with a shotgun blast from behind.
Scudder in Maurice. In the book he's foreshadowed subtly several times; in The Film of the Book, he gets one awkward scene where he (as a random servant) is asked by name to do some menial task.
Mickey in Scream 2is bafflingly revealed to be the killer at the end of the film. Prior to this, he was a minor character who occasionally got into semi-amusing arguments about movie sequels.
The same thing can apply to Roman in Scream 3. He's given a few scenes during the film, but it isn't until the end that you find out he's Ghostface, Sidney's older half-brother, and the mastermind behind what happened in the first two Scream movies.
One of the strangest appearances of a Chekhov's PERSON, in the horror-porn film One-Eyed Monster. Veronica, the old whore who has spent most of the entire movie in an unconscious coked out stupor, saves the day by intercepting and preventing Ron Jeremy's detached alien-possessed member from cumming, causing an incredibly huge explosion that takes out her and the alien. As she puts it "Have you ever heard of what I can do when I'm on my cankles? I can catch a bullet with this thing. (flexing legs back and forth suggestively)" As to the reason for her sacrifice? She's "47 and the world is no place for senior citizens."
JimRash's character in Sky High. We first see him as the love interest's father; he has two lines and then disappears. When he next shows up, it's in the climactic school dance scene, and if the viewer notices him at all (unlikely, as he's in the background and the viewer is probably looking at Kurt Russell), they will probably assume he is just chaperoning his daughter. Then the Patrick Warburton-voiced Big Bad's identity is revealed, and Jim gives an incredibly creepy smirk...
In Devil, the old woman goes largely unnoticed thanks to the other elevator occupants' behavior and her dying second. At the end she's revealed to be the titular Devil. Also Tony, who at the end is revealed to be the man who accidentally killed the main cop character's wife and kid in a car accident five years earlier.
In The Devil's Advocate, the annoying Florida reporter - who seems determined to document when Kevin finally loses - ends up being one of the many faces of Satan - and he's successfully able to manipulate Kevin twice.
In Predators, the character of Edwin seems like an immediate candidate. Initially, he stands out as a normal person in a group of badass killers, leading the genre savvy Royce to wonder why Edwin was kidnapped along with them. For much of the film, he lives up to his normal reputation, with the only major break in action being briefly stabbing a Predator in a moment of desperation. Brilliantly, Royce almost manages to subvert the trope by theorising that the Predators took Edwin in order to add a "human" element to the group, that may end up dividing them. He's almost right...
In Exam, Deaf ends up being eliminated early, after White tricks him into eating his paper. Later, Deaf is revealed to be the CEO who organized the Exam to begin with. Why? To see which person had the good moral character to help him spread the cure for the plague that takes place during the film's setting.
Mallrats: All Willem wanted to do was to see the sailboat in that picture. Through a chain of circumstances, however, his frustration at being unable to see the sailboat is what ends up saving the day for TS and Brodie.
The Oracle in The Matrix, who seems like a harmless old lady when she first shows up in the first movie but turns out to be the co-creator of the Matrix itself.
After Dr. Gordon survives his game in the first Saw movie, he is never seen again until Saw 3D, where he becomes an in-movie Chekhov's Gunman as he's first seen cauterizing his amputated foot and later shows up at a gathering of Jigsaw survivors. At the end of the film, it turns out he had been nursed back to health by Jigsaw, given a prosthetic foot, and trained as an apprentice, turning out to be the only truly faithful apprentice when Hoffman kills Jill Tuck and Gordon, acting on Jigsaw's orders, captures Hoffman and leaves him to die in the same room he himself had survived in the first film.
In Pulp Fiction you can (Just barely) hear Vince and Jules conversation in the background during "Honey Bunny" and "Ringo"'s conversation before they rob the restaurant. You can also see Vince walking to the bathroom in the background (you have to look closely.)
In the 2008 Thai action movie Chocolate, the protagonist is a autistic savant girl who becomes a tremendously skilled martial artist by observing muy thai boxers and watching martial arts movies. About halfway through the movie, a severely mentally handicapped boy is seen amongst the villains and told to leave them while they conduct business. Guess who the most challenging opponent to the main character turns out to be?
Sucker Punch examples contain Wiseman who turns out to be a kind bus driver that gives Sweet Pea an alibi to evade the police, the young British soldier that catches Sweet Pea's attention in the trench warfare sequence who is a passenger on the bus that she rides on at the end of the film and The High Roller who is actually the doctor who performs the lobotomy on Baby Doll.
In Unconditional Love, the killer turns out to be the window washer, who'd been seen in the background earlier in the film.
In UHF, there's a few apparent throwaway scenes of a bum going around asking for change. Said bum turns up at the very end and manages to help the station at the very last second.
In The Dark Knight Rises, when Blake arrives during the Gotham Stock Exchange heist, he walks over to a construction worker sitting in his cement truck and tells the guy to move so that they can get their police equipment in to block the street. Unfortunately, as soon as Blake instructs the truck driver to back up, crash barriers are deployed and he is ordered to stay where he is. It is thanks to the truck that Bane and his men are able to get out of Wall Street without being touched by the police. When Blake is checking Daggett's construction sites, he runs into this very same truck driver. Blake shoots both him and his colleague after they try attacking him with knives, which reveals that the truck driver had been planted during the heist for the purpose of providing a clear escape route.
GI Joe The Rise Of Cobra: The Doctor states that 20 Neo-Vipers have been created and 19 were present in their base. The 20th is said to be in Washington, and is later revealed to be in the Secret Service.
The deactivated drone that is being repaired on Tower 49.
Sally. Not only do we discover that she's nothing but the "human face" of the Tet, but that the real one (on whom the face is presumably based) was Mission Control for the Odyssey mission.
Jack Harper. Clone Number 52.
Adele Hasn't Had Her Dinner Yet: Spoofed, as are most other tropes in this Affectionate Parody. Nick Carter, an American detective investigating a mysterious case in Prague, pretends he's Nick Ledvina, a Czech-American expatriate who is visiting his old country. Professor Bocek asks whether he met a guy named Ladislav Matejka who is related to Professor Bocek's family. Nick is mildly annoyed that the professor imagines America as a small Bohemian town and says he didn't have the pleasure. When Nick is leaving Prague on a train, Professor Bocek keeps waving and shouts " and if you ever happen to meet Ladislav Matejka..." Then all of the sudden, Nick's Identical Stranger appears out of nowhere, and he says in Czech with a strong American accent: "But that is me, Larry Matejka, a Czecho-American."
Scotty resigns before the mission after vehemently disagreeing with Kirk on using unidentified Photon Torpedoes. Kirk later enlists his aid in investigating Harrison's claims, culminating in Scotty sneaking on board the Vengeance and disabling it to save the Enterprise.
Pavel Chekov manages a surprise save of Kirk and Scotty. Yep, he was still down in Engineering.
In The Hangover, Black Doug mentions that "Marshall is gonna kill me". Four years and two movies later, Marshall appears, played by John Goodman.
Lionel Shrike in Now You See Me.He happens to be Dylan Rhodes, the cop who's been chasing the Four Horsemen the whole damn movie.
A character in Film/Contact who seems to be nothing but a colorful background character eventually is shown to be a domestic terrorist.