So many characters in Star Wars that it would be impossible to list them all. For example, watch the cantina scene from the original film; every last patron has at least a name and a bit of backstory. Some actually get their own story in the anthology book Tales from the Mos Eisley Cantina; several who hadn't been named ended up on the Star Wars Databank, where fans were allowed to come up with their own backstories and names, then vote on which ones were the best. After that, though, they tend to never come up again - the Star Wars Expanded Universe is infamous for this and a few characters do recur, but honestly, most characters in the EU were either named and had a line or two or are entirely original creations who were never on screen. Even fan favorites like Boba Fett and Wedge Antilles, moving on to their own novels and appearing aCamp Lakebottomgain and again, were minor non-mains who still had lines and plot significance.
Remember that nameless red droid with a bad motivator from Episode 4? This trope was mocked in the non-canon comic Skippy the Jedi Droid, where he was a Force-Sensitive droid who used to work for Jabba. He self-destructed on purpose because he could see the future and knew that R2 needed to go with Luke, or even Skippy's Parody Sue skills couldn't stop Reality from Ensuing badly.
Ask any hardcore Star Wars fan about the silver protocol droid Threepio meets in Cloud City just before he gets blasted. Go ahead. We'll wait.
A good example of the difference ascension makes: This is a normal Imperial Guard◊. And here's Kir Kanos◊, who wasn't in the films but has a comics plotline of his own, in what could be a case of the job producing an Ascended Extra.
Several characters from Death Star. The guy manning the Death Star's main weapon, who appears once and says "Stand by"? There's a whole story about him.
Wulff Yularen, originally an unnamed background officer on the Death Star in A New Hope was given one in the collectible card game and then generally forgotten. Come 2008 and he's probably the largest Republic non-Jedi, non-clone military figure in Star Wars: The Clone Wars for the first two seasons.
In The Lord of the Rings novels, Arwen appears only briefly in the main narrative with the majority of her and Aragorn's story relegated to a chapter in the appendix. In Peter Jackson's version, she is featured prominently in all three films and replaces several minor characters, like Glorfindel who aids Aragorn and the Hobbits as they flee the Ringwraiths. Not only does Arwen replace Glorfindel, she's given the additional task of personally carrying Frodo upon her horse, and her Crowning Moment Of Awesome — "If you want him, come and claim him!" — is a unique addition to the film. In the novel, Glorfindel places Frodo on his horse and then stays behind with Aragorn and the Hobbits to do what he can to slow the Ringwraiths' pursuit, at which point the narrative focus shifts entirely to Frodo.
In The Hobbit, several characters go through varying levels of this. Azog, The Pale Orc, who had been briefly mentioned (and had never appeared) in the books became the main villain of (at least) the first movie. Radagast, the Brown Wizard, was also promoted from a bare mention to a major, plot-important character, though to a lesser degree than Azog.
Alien introduced The Space Jockey, which was an ominous creature that appeared to be the pilot of the Derelict. It had been killed and fossilized before the events of the first film, and it was at the centre of much speculation among the fandom, as well as many non-canon stories in the EU, for over 30 years before its identity was revealed in Prometheus.
The Meganulon (Giant dragonfly-like monsters) from Rodan were only minor characters in the film. Later, they would become the major villains in Godzilla vs. Megaguirus.
Scabior in the Harry Potter and the Deathly Hallows Films. He appears very briefly in the books when he and other snatchers capture the trio and take them to Malfoy Manor. After that, he disappears from the plot, though there is speculation that he is killed by Voldemort after the trio escapes with Dean, Luna, Griphook and Olivander. He appears much more in the films peppered about to the point where he even appears in bits of Part 2 that he didn't appear in during that part of the book.
Ocean's Eleven: Denny the whale (uber-producer Jerry Weintraub) has a tiny cameo in the first film, causes the second film by unwittingly bragging about the heist to two European master thieves, and makes amends in the last film when he convinces the other big spenders to leave Bank's casino. Also, the mansion seen in the last movie is one of Jerry's Big Fancy Houses.
Playing with this trope, Guy Fleegman in Galaxy Quest was just another Red Shirt who got killed in the original series. But after going on a real space adventure with the crew gets his own starring role in the new Galaxy Quest series.
In The Crow, there's that one little girl whom Eric helps that has a name similar to his dead fiancee's, yeah that one... Well, she gets a name change (Sarah), a big role in the film (the bestest buddy of Shelly and Eric), and her mom's involvement with Funboy is upped. From there, she becomes the love interest in the sequel and once again is in the series.
Both the Worm Guys and Frank the Pug in Men In Black were one-off jokes in the first movie, but had expanded roles in the second movie and the series.
In Spider-Man 3, Elizabeth Banks and Ted Raimi receive billing after their smaller roles in the previous films led to slightly bigger ones in the third.
Every doll besides 9 and 5 in the film 9. In the original short film, the remaining seven dolls did appear, but only for a few seconds at the end of the film. The feature film takes all of them and gives them larger roles and actual personalities (as well as complete character make-overs).
Micheal Jai White, who played a small part as an unnamed soldier in the first Universal Soldier, later played the Big Bad in Universal Soldier: The Return.
"Happy" Hogan, Tony Stark's bodyguard in Iron Man played by director Jon Favreau, has considerably more lines in the second movie; in addition, he gets to beat down a guard, ram Ivan with his car, and has Black Widow (aka Scarlett Johansson) put him in a headlock with her legs.
Iron Man 2: Jack White was originally hired as the food stylist who prepared and served the salmon carpaccio to Vanko in the aircraft hangar, but he ended up appearing in several scenes as Jack, Hammer's assistant.
In Kung Fu Panda, the Furious Five were little more than window-dressing. In Kung Fu Panda 2, they have a much more active role, particularly Tigress.
Martin Clunes plays a supporting character in Saving Grace, Dr. Martin Bamford. The character was expanded into two prequel tv films explaining his background. Sometime after that, the character was given a retool, and became the main character on the series Doc Martin.
In the original Bambi Ronno is an unnamed stag who fights over Faline with Bambi. In the midquel he's a fleshed out buck and the central antagonist of the movie.
In The Whisperer In Darkness by H.P. Lovecraft, George Akeley, Henry Akeley's son, is a very minor character who is only mentioned a couple of times in his father's letters, is never seen in person, and clearly survives the events of the story. In the movie based on it, he is a key liason between Henry and Professor Wilmarth early on. Unlike the story, he is strongly implied to be caught and killed offscreen by the Mi-Go when he tries to deliver the Black Stone to Wilmarth.
In the original Rocky, Little Marie appears in one scene where Rocky walks the 12-year-old home. Three decades later, Marie shows up in the final film Rocky Balboa as the lead female character and love interest (after Adrian's death).
Ulla in the original film The Producers is nothing but a walking sex joke, barely speaking any English and only showing up to dance occasionally. In the musical, she's made into a fully developed character who speaks English more or less fluently, and becomes Leo's love interest.
In the third movie in the Halloweentown series, Halloweentown High, Ethan (a warlock) was a minor character who was mainly a rival to Marnie but who wasn't important to the story. In Return to Halloweentown, he is a love interest to Marnie. This may be because Lucas Grabeel, who played Ethan, had just been in High School Musical and Disney was trying to promote him.
The Greatest Gift, the short story that inpsired It's a Wonderful Life, says nothing about Mr. Potter other than the fact he owned a photography studio. The movie turns him into a Corrupt Corporate Executive who owns nearly all of the town and seizes a couple opportunities to ruin the protagonist's life.
In the Tintin comic The Secret of the Unicorn, the minor character Ivan Sakharine is a pushy but ultimately harmless model ship collector who is briefly suspected of stealing Tintin's model of the Unicorn. In the film adaptation of the same comic, however, he is the main villain and the descendant of the pirate Red Rackham. Ironically, in the comic Sakharine is later attacked by the real thief, and if a brief cameo appearance in Red Rackham's Treasure is anything to go by he eventually made his peace with Tintin.
Ariel's sisters only appeared at the beginning to sing a song and made only one appearance afterward. The TV series and the prequel '"Ariel's Beginning'' gave them bigger roles and established personalities.
In the original Dracula novel the Brides only make two appearances and are unimportant side characters. Van Helsing greatly expands their roles, making them Dracula's right hand women and powerful combatants.
In the first Toy Story film, Woody and Buzz were the real focus of the film and all the other toys were relatively minor characters. The second film gives much more screen time to Slinky, Rex, Hamm, and Mr. Potato Head. And likewise, Mrs. Potato Head only appeared in the beginning and end of the second film while Barbie was only in one scene roughly past the middle; they both had much bigger roles in the third movie.
Jiminy Cricket was originally a nameless cricket who, in the original book, got squashed by Pinocchio in one chapter (and came back as a ghost much later). In the film, he becomes Pinocchio's friend and adviser and has since appeared in countless other Disney projects.
Q receives this treatment in a couple of the James Bond movies. While usually he's just there to give Bond his new devices and then takes off, in Octopussy he actually helps Bond when he raids the villain's hideout at the end and travels to Isthmus City to help Bond out in Licence to Kill after 007 goes rogue to get revenge on Sanchez, even ignoring Bond's request that for his own good he should return to London.
Also, M in every other Bond film just gave Bond orders and info on the mission. In Skyfall, she was given a backstory and protecting her was the driving force.
In How the Grinch Stole Christmas! (both the original book and cartoon), Cindy Lou Who is just a character who exists to show that the Grinch is a liar, with only one line. In the movie version, she practically becomes the co-star, playing a more vital role in the plot than any character other than the Grinch himself.
In the first Shrek film, fairy tale characters like the three pigs, Pinocchio and Gingy were joke characters and mainly appeared in the background. In Shrek 2, they have much bigger roles, helping to break Shrek, Donkey and Puss out of prison and fighting off the Fairy Godmother in the climax. They have since had decent roles in further Shrek films and specials.
In The Wizard of Oz, the Wicked Witch of the East didn't really even rate as an extra, as she's only seen as a pair of feet sticking out from under Dorothy's house. In Oz: The Great and Powerful, she's the one who initiates all the trouble: murdering the previous king, sending winged baboons against innocent villages, and maliciously tricking her own sister into hating Oscar and becoming the Wicked Witch of the West.
Roadblock isn't exactly one of the leading members of the G.I. Joe team, but plays a big role G.I. Joe: Retaliation.
In The Hangover trilogy, Leslie Chow goes from a minor role in the first, to a bigger supporting role in the second, and basically the main antagonist in the third.
Pinhead of the Hellraiser series was a minor component in the original novel The Hellbound Heart. Come the film adaption, the more prominent Cenobites from the novel either couldn't speak (Butterball) see or speak (Chatterer) or had their motion limited by their complicated makeup (Female Cenobite). So, Pinhead took the lead, and quickly became the face of the franchise.
In Rock-A-Doodle, the Grand Duke of Owls is the main antagonist and the cause of all the trouble in the movie. In the play Chanticler by Edmond Rostand, the Grand Duke is only a minor villain - instead, the main villains are the Blackbird and the rival rooster who fights Chanticler. The former receives Adaptational Heroism while the latter is Demoted to Extra.
Back to the Future: Biff Tannen's 1955 gang plays a bigger part in Part II. In the alternate 1985, it's shown that they're still working with Biff when he's become insanely wealthy, and when Marty is back in 1955, they chase him into the gym where Marty's other self in the first film is playing, and they go backstage to fight "him", so Marty has to stop them in a way that won't impede upon the first film's chain of events.
In the original Sleeping Beauty, King Stefan was just a Bumbling Dad of a king who had about one scene. In Maleficent, he's a guileful, skilled warrior who was the titular character's childhood friend, and whose actions are what started the plot of the movie in the first place.
Aurora herself is this. Despite Sleeping Beauty being about her, she lost screentime to the fairies and Maleficent in the first half of the film, and was comatose and mostly absent in the second half. In this film, she's much more active, seeking out the truth of her curse herself, riding off to her father's castle, and has an important role in the final battle.