Small Role, Big Impact

""It seemed that my lot in life was to either have big parts in small films or small parts in big films.""

- Bruce Campbell, If Chins Could Kill: Confessions of a B-Movie Actor

Small Role, Big Impact is when a minor character (sometimes a very, very minor character) who, through his or her actions or words, has an impact on the story far, far beyond what such a minor character ought to have normally. Note the difference between this and a One-Scene Wonder, a character who has limited screen time but their actions or words have a huge impact on the audience.

This can occasionally overlap with One-Scene Wonder, Posthumous Character, The Ghost, Spear Carrier.

Anime & Manga

 * Lord Asano is never actually seen in Princess Mononoke, but those are his samurai who are dangerously close to conquering Iron Town, and it may also have been his men that Ashitaka saw brutalizing the countryside before.
 * The Emperor in Onmyoji is hardly in it bar a couple of scenes in which he does very little that's useful, but it's his rejection of Suke-hime that leads to most of the villain's attempts to kill the imperial family via her angry father and her eventual transformation into a demon.
 * Admiral Robert J. Hanner, United Planets Space Force (ret.), from Irresponsible Captain Tylor appears only about four times in the 26 episode series, and only once in a speaking role. However, directly or indirectly, he's responsible for Tylor becoming a starship captain, the Soyokaze crew getting demoted, the war with the Raalgon being able to conclude without additional bloodshed, and his death sparks a Heroic BSOD from Tylor.
 * Dying Breed rarely appear in Beck, but they inspire both Ryuusuke and Koyuki to push the band to great heights, and  causes a Heroic BSOD from almost the entire music world. Not only that, but rumors of an unreleased song of theirs drive a huge portion of the plot.

Film - Animated

 * The Little Green Men in Toy Story 3.
 * Edna Mode from The Incredibles. She appears three times, all in the first half of the film. She's the one who alerts Helen to her husband's moonlighting hero work, and convinces Helen to go track Bob down.
 * Ellie in Up.

Film - Live Action

 * The Blind Seer in O Brother, Where Art Thou? is only in the movie for a couple of minutes or so at the beginning of the film, and for less than a minute at the very end, but his initial scene sets up the adventures of the main characters.
 * In the 1999 movie version of A Midsummer Night's Dream, Hippolyta (the vanquished Amazonian Queen marrying Duke Theseus) is mostly a background figure. However, in the movie when the Duke and his party discover the lovers in the morning, Hippolyta pulls Theseus aside and has some words with him, which go unheard by both the audience and other characters. Afterward Theseus announces that the lovers may marry according to their own wishes, rather than according to the decree of their families.
 * Ricardo Montalban once said that he almost passed on coming back for Star Trek II the Wrath of Khan because, as it is written in the script, Khan is actually only onscreen for about fifteen total minutes over the course of the entire movie, and his actual spoken dialogue is pretty minimal as well when compared to the main characters. But then he realized, as he read the story, that Khan's impact on the other characters is present on every single page of the script, and immediately agreed to reprise the role. (It's worth noting that Khan's name hadn't been put in the title yet.)
 * In The Third Man, the chillingly evil Harry Lime is at the center of the plot but appears for less than 10 minutes on screen. Orson Welles plays him as just a normal guy you wouldn't look twice at, and takes three seconds in a search-light and a somewhat sheepish 'you caught me' grin to completely upstage Joseph Cotten's excellent performance and steal the film.
 * Jack Palance had a film career of 50 years and over 70 movies, but when he died in 2006, one film role consistently stood out in all the obituaries and tributes dedicated to him: the role of the taunting, smiling hired gun Jack Wilson in Shane. Palance's Wilson is widely regarded as the definitive Western bad guy. Total screen time: eight minutes. Total words spoken by Wilson: less than fifty, but he makes the most out of two of them: "Prove it."

Literature

 * In A Song of Ice and Fire, Balon Greyjoy barely has any time interacting with the POV characters, and appears in only small parts of two chapters in the second book, and doesn't reappear before dying off screen in the third. However his decision in the second book to go to war with the North rather than joining them effectively ends the chances of the main characters to win the war they're fighting.
 * Mirri Maz Duur only appeared in four chapters of A Game of Thrones, including the one where she is killed. The only POV character she interacts with is Daenerys. However, her actions become not only the driving force behind much of Daenerys's story arc, but also the reason that dragons (and in turn, stronger magic), have returned to the world.
 * Hannibal Lector from Silence of the Lambs. He's a minor character in the novel (and in the film he's on screen for only fifteen minutes or so), and yet he drives the plot forward on several occasions all by himself.
 * Lily Evans really isn't in much of Harry Potter, but her actions drove the entire characterization of, and her Heroic Sacrifice set up the entire plot.
 * Narcissa Malfoy in the latter Harry Potter books. In terms of facetime and notoriety, she takes backseat to her husband and son and mostly just another snobby wizard supremacist. However, in the sixth book, her binding Snape to the Unbreakable vow is ultimately responsible for the climax of the story. And in book 7, her willingness to lie to Voldemort about Harry's death is what gives Harry the chance to end him once and for all.
 * Sasha in Warriors: The New Prophecy. Although she only appears once or twice, she mothered
 * Socks and Ruby in the Warrior Cats manga The Rise of Scourge. Both were mean to Tiny, causing him to run away and become Scourge. Also Tigerclaw, but only in the manga.
 * The Maltese Falcon: General Kemidov is The Ghost, but even before the story begins, when Gutman wanted to buy the McGuffin, he realized that it would be important and replaced it with a Mock Guffin that the gang found very easy to stole, making him the real Magnificent Bastard of the story.
 * The Lord of the Rings books are full of this. You have things like Erkenbrand, a Marshall of Rohan leading the troops that Gandalf collects to save everyone at Helms Depp, or Ghan-buri-Ghan, a Noble Savage tribesman who leads the Rohirrim around an ambush so they can arrive at the Battle of the Pelennor Fields in time and at full strength.
 * Arianna Ortega in the Dresden files interacts only with Harry, and appears in a grand total of three chapters before biting it. Her plans result in Harry damning his soul forever, and sets the plots for book 12 and 13 in motion.
 * The Story Within a Story in Hans Brinker or The Silver Skates about a boy plugging a dyke with his finger and preventing a flood is more famous than the actual story of Hans Brinker and his skates.

Live Action TV

 * In Lost, Jacob never appeared onscreen until the end of the fifth season... in an episode in which he was killed at the end. In the next season he appeared just a handful of times as a ghost or in flashbacks. Nevertheless, he is one of the major characters in the Myth Arc of the series.
 * Phil Davis appeared on Merlin in a guest spot that lasts no more than five minutes. In that time he mortally wounds, a major character who had been on the show since the beginning, and changes the entire course of the show.

Video Games

 * Bane in Batman: Arkham Asylum. His only appearance is a brief, yet epic confrontation, but  is basically the catalyst for the entire game's plot.
 * The "Oriental Gentleman" from Grand Theft Auto III, an unnamed man who just so happens to be a prisoner in the same police convoy the player character is in - the fact that the game consists of typical Grand Theft Auto fare rather than the player being behind bars for the whole game (or at least a few minutes, considering the series) is due entirely to the fact that the Colombian Cartel decided to hold up the convoy and take this one prisoner.

Theatre

 * Joe in Show Boat. It helps that he has one of the best Broadway songs ever written, "Ol' Man River."
 * It's easy to forget that Tybalt from Romeo and Juliet is only in about 3 scenes in the play. But as Isaac Asimov pointed out succinctly in his analysis of the play, without Tybalt, the rest of the play's events would never have happened.
 * Jessica, daughter to Shylock in The Merchant of Venice, has very few lines, but the whole play hinges on her elopement and her conversion to Christianity, which drive her dad over the edge. Whether she's seen as a "good" or "bad" character is a key decision when staging the play, and directors tend to give her plenty of extra stage time to pray in Hebrew or look tragic. Lorenzo could also be seen this way--besides being the boy who steals Jessica, he has one of the play's best soliloquies ("How sweet the moonlight sleeps upon this bank...")