Stuart has been hanging around the group since Season 2, showing up a couple of times a year but never really significant. Season 6 practically upgrades him to main cast, at the least supported by Word of God that he will have an expanded role. While he didn't stay in the main cast listings for long, unlike Leslie Winkle he still shows up more often than before and usually with a significant role involved.
Leslie Winkle start from this to Demoted to Extra. She showed up three times in the first season, and her character was liked enough that the production team announced her as becoming a regular early in the second season. After about four episodes in, they quickly realized that she worked largely because she was a staunch, stubborn Arch-Enemy to Sheldon with no real room for her own stories or even Character Development. They quietly put her character aside and she shows up more sporadically since.
Harmony was originally one of Cordelia's cronies who was in the second half of the pilot and got bitten by a vampire at the end of Season 3. She shows up again in Season 4 as Spike's vampire girlfriend. She later showed up in the Buffy spin-off, Angel, eventually promoted to main character. She was the only Buffyverse "extra" to be promoted to main cast status. note However, her actress Mercedes McNab was not the only "extra" actor to be promoted, as Andy Hallett played an extra on Buffy before joining the cast of Angel. She also holds the distinction of being the only character (and actor) to be part of the Buffyverse for its entire run, appearing in both the original unaired pilot and the last episode of Angel.note Though David Boreanaz did appear in a deleted scene in the unaired pilot.
Danny Strong was credited as an unnamed extra on "Buffy" for quite some time (again, starting in the unaired pilot!) before even obtaining the name "Jonathan". He became a minor recurring character and one of the three main villains in season 6. And in Season 7 it got to the point that The First thought him important enough to be included in its evil plan.
Anya was originally intended as a one-shot villain for "The Wish." She returned for the Sequel Episode "Doppelgangland", and continued in a recurring role before being promoted to the opening cast in season 5.
Eliza Dushku's Faith originally started out with a planed minor role in the series, similar to Kendra's, but both the audiences and Joss Whedon became fans of Eliza and her character and she became a much more important to the story.
Amanda first appeared as a girl getting advice from Buffy while she worked as a school counselor, but later was revealed to be a potential Slayer.
Tara Maclay was originally supposed to be on the show for a couple of episodes, to help justify the growth of power in Willow by teaching her magic. But after Joss Whedon and others on the crew noticed the chemistry between her and Alyson Hannigan, she was Promoted to Love Interest, essentially becoming an extreme case of Fake Guest Star (appearing in more episodes and receiving more development than Riley or Oz, who were main characters) and helped define one of the most well-known same-sex relationships on Network Television.
Family Matters: Harriette Winslow, the elevator operator at the Chicago Chronicle, was originally a recurring character on Perfect Strangers (a comedy starring Mark-Linn Baker and Bronson Pinchot), usually to give one-liners or provide superficial support to the storyline at hand. Then, her husband, Carl (a police officer with the Chicago Police Department) began appearing a couple of times ... and soon enough, someone got the bright idea that Carl and Harriette and their family (the children were referred to a couple of times) might make an interesting program. Hence, Family Matters.
During the course of the series, several one-time minor characters were ascended to starring roles, virtually taking the emphasis completely off the Winslow family's dynamic in the process. However, much of this had to do with how well the actors playing these roles went over with the audience:
Waldo Faldo. Also introduced midway through the first season, in the episode "The Party" (aired February 2, 1990). One of the many guests at Eddie's house party (held without his parents' permission when they're not home) is a "Boy in Towel." The "Boy in Towel" was a background character who showered in the Winslows' shower and acted like an idiot. Just like Waldo did late that fall ... when he was given his name and semi-regular status. By Season 3, Shawn Harrison's portrayal of the character was so popular he earned his spot in the opening credits and began appearing in virtually every episode.
Myra Monkhouse. Like Urkel and Waldo before her, Myra was meant to be a one-time diversion for Urkel so that her cousin — who happened to be Laura's boyfriend, Ted Curran — could have her without interference from Urkel. Ted was dropped a couple of episodes later. Myra remained for almost the entire rest of the series (until Michelle Thomas fell ill with her ultimately fatal illness in early 1998) and was given full billing.
Game of Thrones: Ros, a Canon Foreigner, was originally intended only for Tyrion's introductory scene, but the producers liked the actress and expanded her character into additional scenes, eventually turning her into a fairly major supporting character.
Parodied in a Ferrell-era sketch on Saturday Night Live with NBC making an adaptation of the New Testament. As they don't have time for a script, they just ad-lib (something that Will Ferrell, playing Jesus, doesn't feel comfortable doing). When the actors can't decide how their characters should know each other, they just say they have a mutual friend named Barry. By the end of the sketch, Barry gets an "actor" (one of the boom-mike guys), Jesus betrays Barry, and (the film now taking place in a college dorm) Barry kills the Devil.
This seems to be SNL's policy regarding Featured Players: they are billed after the reperatory cast, and generally get less screen time, and if they manage to make enough of an impression to stick around for two years, they are promoted. For some, like Eddie Murphy above, or Amy Pohler, they are so well-liked that they are promoted early, even midway through their first season.
Walter aka (Engaging Chevron Guy). Something odd as he never really gets a main character spot. He does get a big nod in the end of the 200th episode. However in the fandom, Walter (and his actor) have been used as stars in promos and skits. A promo for the aforementioned 200th episode has him going to ask his castmates, producers, writers and directors for his part on the special, only to realize none of them recognize him despite him having been on the show since the pilot.
When Daniel was written out of the show, the character (Jonas Quinn) that enables the team to find out what really happened to Daniel was supposed to be a one-shot character. However, the producers decided to use him as Daniel's replacement for Series 6. This is an example of last-minute decisions back-firing. Due to the role the character originally had in Daniel's death, and the fact the fandom was angry at losing Daniel from the show, Jonas struggled to be accepted by the fandom. When Series 7 was commissioned, Daniel returned to the show and Jonas was written out.
Vala progressed from single-episode guest star in Season 8, to recurring guest in Season 9, to full-time cast member in Season 10.
Dr. Rodney McKay, played by David Hewlitt, had a few SG-1 episodes where his role was little more than to be the arrogant scientist that makes Samantha Carter look brilliant. When Atlantis was commissioned, David Hewlitt auditioned for the role of the scientific character and nailed it so completely, the producers decided to scrap the proposed character in favour of the already-existing McKay character.
Dr. Carson Beckett was originally only written into a few episodes throughout season 1, but he was such a big hit that the writers put him in several more. In later seasons, he's promoted to one of the main characters.
Major Lorne appeared in a single episode of SG-1. Starting from season 2 of Atlantis he became a major recurring character that is designed to act as back-up to the main cast.
The pilot of Star Trek: The Next Generation features an unnamed, curly-haired conn officer played by Colm Meaney. That character would reappear on a regular basis in subsequent episodes as a gold-shirted (the 24th-century equivalent of Red Shirt) transporter operator. That operator eventually got a name and rank, Chief Miles O'Brien, and ended up becoming a semi-regular character with a wife and family who interacted regularly with the main cast. When it came time to create a spinoff of TNG, O'Brien went even further to become a main character in Star Trek: Deep Space Nine. Miles O'Brien is the embodiment of a character working up from the Red Shirt Army to donning fullblown Plot Armor.
Damar ascends from the role of one of Dukat's nameless henchmen to that of national hero for Cardassia in the final episode. The main difference between O'Brien and Damar is that (at least according to Word of God) Damar was always intended to play a major role in the Dominion War, while O'Brien was only a nameless extra in the TNG pilot (Meaney got the part as a sort of consolation prize; he was runner-up in another role).
Garak was only intended to appear as a guest star in a pair of episodes, but Andrew Robinson's performance impressed the producers so much that he was brought back for more and more episodes, to the point that he has a pivotal role in the final string of season 7 episodes.
Admiral William Ross was supposed to be the typical Spear Carrier, but Barry Jenner's portrayal of an atypical sane Starfleet admiral made the producers bring him back. He managed to displace Sisko as the face of the Federation war effort by the end of the series.
Many bit characters from the Star Trek shows have become major players in the tie-in novels, even starring in some of the book-only spinoff series. Star Trek: New Frontier made regulars out of one- or two-shot Next Generation characters Shelby, Selar, and Lefler. Starfleet Corps of Engineers features guest engineers like Sonya Gomez, Duffy, and Stevens. A major character in the Star Trek: Deep Space Nine Relaunch novels (Vedek Yevir) is based on an extra who appeared in one scene and was only seen from behind. Star Trek: Ex Machina, a sequel to Star Trek: The Motion Picture, bases most of its supporting crew characters on faces (and alien masks) in the crowd from a crew-assembly scene in that film. Meanwhile, Agents Lucsly and Dulmur of the Department of Temporal Investigations, who were minor joke characters with less than four minutes of screen time in one episode of Deep Space Nine, have been the focus of several works of short fiction, culminating in a series of novels about the DTI.
An averted example is Star Trek: Voyager's Tom Paris, who was originally supposed to be the same character as Nick Locarno, a one-off character from Next Generation. Because the producers would have to pay royalties to the episode writer, the character was given a new name, but kept the same actor, personality, and backstory. (The writers say it's because Locarno was irredeemable, but that rings pretty hollow when the only change to the backstory is that where one person died in the Locarno version of the incident, three died in the otherwise-identical Paris version.) Tom's father's picture of Tom as a cadet is even a still of Locarno taken from that episode.
The Trek producers are notorious for that. Locarno-into-Paris is the big one, but other instances of same-actor, same-personality, new-name Expy characters who are absolutely, positively new characters who are definitely not just renamed versions of one-shots they don't want to pay royalties for, no-siree include Taurik/Vorik and Gul Macet/Gul Dukat. Also, while it's not quite as big as going from one-shot to central character, the Romulan commander Taris returns in a much meatier role a few seasons later in a second episode… only now we're calling her Toreth. Though not with the same actor because she had to get younger, not older, Enterprise's T'Pol was originally planned to be T'Pau (whom we would eventually see in a three-parter.) Also, Michelle Forbes was intended to reprise Ro Laren but declined, twice. DS9's Kira and Voyager's Torres were both originally planned as her. Not that Ro was just an extra.
Interestingly, while in the show Gul Dukat was simply a renamed Gul Macet, in the Expanded Universe, Macet gets to properly ascend in his own right... and as very much his own person. Instead of making him pretty much the same as Dukat so they could proceed as if the popular villain hadn't died in the show's Grand Finale, it turns out that he's the most decent high-ranking Cardassian you'll ever meet, is not guilty of anything like the horrors Dukat was revealed to have committed during the Occupation, goes on to make a full Heel–Face Turn, and is just generally the opposite of the utter bastard his cousin is (looking less like him is even the reason behind his facial hair.) Oh, and he gets a first name, too! In his latest-set appearances, Ro Laren will even work with him.
Painfully averted in Star Trek: Voyager by Lt. "Extra-Man" Ayala. Ayala appears in 120 episodes out of 167, in all seven seasons. He speaks in exactly four of those episodes, and is only credited twice, never with a name. The mere act of establishing a name for the actor took some detective work. He's achieved a certain amount of Memetic Badass status among the fandom for simply managing to surviving all seven seasons.
The Gorn are a whole race of these - towering, reptilian aliens who were the primary focus of the Original Series episode 'The Arena', and had brief cameos in an episode each of the Animated Series and Enterprise. Despite these obscure beginnings, they crop up astonishingly frequently in games and other supplementary material, even becoming a playable race in Star Trek: Starfleet Command and Star Trek Online.
Gonzo first appeared in the Jim Henson and Ed Sullivan Christmas SpecialThe Great Santa Claus Switch in a small role as a creature living in the villain's cigar box named Snarl. The puppet was repurposed for usage on The Muppet Show and the rest is history.
Rizzo the Rat was also a minor character originally, going on to fame and fortune as Gonzo's sidekick. And Ms. Piggy, as originally conceived, was meant to be a bit player.
Lew Zealand was originally intended to be a one-shot character. Following his first appearance, the Whatnot pattern he was made from was altered into a more permanent puppet.
Uncle Deadly was a memorable character who only appeared every once in awhile on The Muppet Show. After two decades of not being used anywhere, and having only made background appearances in the movies (in large crowd scenes), he was given a featured role in the 2011 film as one of the villains henchmen, and now has a recurring role on the 2015 series, being in charge of wardrobe.
There's also Chip, an egg-headed humanoid Muppet, having originally mainly appeared on The Jim Henson Hour before mostly being demoted to background roles during the 1990s and 2000s, finally having another major role in The Muppets as the studio's I.T. guy.
Sesame Street's Elmo was originally a nameless background character, but after Kevin Clash took over the role, he became a more regular character, and eventually received a recurring 15-20 minute sketch.
Thanks to The Next Iron Chef, Michael Symon, initially a challenger on Iron Chef America , ascended to a position of Iron Chef. In season two, Jose Garces, who had defeated Iron Chef Bobby Flay, joined him.
Battlestar Galactica (2003) had several characters who initially appeared in one-off or recurring roles, only to gain greater prominence in later seasons:
Helo was intended to be a one-shot character from the miniseries, who was assumed to have died offscreen after giving up his space on Boomer's Raptor for Baltar. However, the producers not only brought him back, but his B-plot in the first season (traveling around Caprica with Sharon) becomes pivotal to the entire resolution of the series.
Louis Hoshi starts out as a rarely-seen communications officer on the Pegasus. He gets more and more appearances in later seasons, eventually taking up Dualla's job after her death. By the time the series finale rolls around, Hoshi is given the role of ''Admiral of the Fleet' by Adama after his team leaves to go on the rescue mission, solely because he is essentially the only other capable command officer left to run the ship.
Captain Kelly starts out as a minor character who mans one of the flight pods in the miniseries, then later shows up for two episodes in the second season to help the command staff after Bill Adama is shot. He later gets a major role as the bomber during the trial at the end of the third season, and even had a redemption arc/Heel–Face Turn in season four during the mutiny onboard Galactica.
Racetrack was intended to be a one-shot character who would appear in the final episode of the first season. However, Leah Cairns' performance impressed the producers so much that Racetrack essentially became the de facto Raptor pilot shuttling major characters around for most of the series, and she later became a Chekhov's Gunman by posthumously destroying the Cylon colony with nuclear missiles in the series finale.
In Doctor Who, the character of Wilfred Mott was initially a brief 2-minute cameo for Bernard Cribbins in the 2007 Christmas special. However, due to the death of the actor playing Donna's father, the character later returned as her grandfather in the 2008 series. His role continued to be increased to the extent that he was promoted to main companion status for the 2009 Christmas and New Year's specials, and caused the Doctor's regeneration.
Donna, who appeared in the Christmas Special following series 2 and then later became The Doctor's companion for all of series 4.
Rory Williams had a minor role as Amy's kind of boyfriend in "The Eleventh Hour". By "The Vampires of Venice" later that series, he became a full-ish companion, who was Promoted to Opening Titles from "A Christmas Carol".
From the classic series:
Jamie McCrimmon was supposed to be a one-off character for The Highlanders, but the team liked him so much that they re-filmed the ending of the serial to keep him. He ended up becoming the longest running companion, appearing in all of the 2nd Doctor's stories apart from The Power of the Daleks and The Three Doctors.
Colonel Lethbridge-Stewart from The Web of Fear was supposed to be a one-off character. Then they decided to bring him back for The Invasion and gave him a promotion, and he became one of the most important characters of the Third Doctor's era, and quite possibly the entire series. If you include multi-doctor episodes like "The Five Doctors", he appears alongside every doctor of the original series, and is mentioned by name by the 10th and 11th doctors. And in the Expanded Universe he has met 8 and 10. Even when the actor died his presence is still felt by his daughter Kate.
Vicki, Elena's aunt Jenna, and arguably Caroline. She had a fairly large role in the books, but on the show she's an entirely different character.
Elijah as an extreme example. Julie Plec admits that Daniel Gillie's performance and the pull of Elijah's character inspired the writers to make the Originals un-killable to keep him around. And he was originally just a "foot soldier" before being made into Klaus's brother.
Toshiko of Torchwood was seen ever-so-briefly in series one of the new Doctor Who first. As an extra, she was a doctor. As a full time Torchwood character, she was explained to have been impersonating a doctor because the actual doctor (fellow Torchwood member Owen Harper) had been too hung over to work.
An extreme example of this is Mrs Miggins from Blackadder. In the second series she was an unseen character mentioned two or three times as a throwaway joke. She became a regular character in the third series.
Red Dwarf has various examples. Kryten was a one-episode character who wouldn't have been included originally as one of the writers wanted to avoid robotic characters. He returned in Series III as a main character (although played by a different actor).
Captain Hollister, another first-episode character and posthumous flashback regular from the first two series. He was brought back for Series VIII, in which he appeared in all but one episode. Unlike Kryten and Kochanski, the same actor portrayed the Captain.
Glee seems to live off of this, especially in the second season.
We'll start with Brittany, though technically she ascended in the first season. Heather Morris was a back-up dancer for Beyonce's "Single Ladies" live performances and was initially hired to teach the choreography to the cast. They gave her the role, intended to be just a recurring background dancer, without even an acting audition. It turned out that she had great comic timing, and after a gradual increase in lines over season one, she was promoted to the regular cast by season two, and is also a major player in one of its subplots.
Similar to Britanny, Santana was little more than a background character in the first few episodes only getting a few lines here and there but eventually started getting more and more focus, probably due both to actress Naya Rivera's ability to sell the part of bitch real well and her chemistry with Heather Morris making them a great comedic duo.
Mike Chang went from a minor character in season one to getting more and more focus in season 2. His actor Harry Shrum will promoted to main cast member in season 3.
Dave Karofsky. He was a two dimensional, stereotypical, homophobic highschool bully but has now been promoted to having his ownsub-storyline.
Lauren Zizes was a randomly-appearing homely looking girl who most fans and characters didn't bat an eye at. Suddenly, halfway through the second season, she became an Eleventh Hour Ranger, Puck's new love interest, and an extreme Badass in one fell swoop.
In The Sookie Stackhouse Mysteries novels, the character of Mary Ann originally named Callisto pretty much just showed up at a party, made a few comments and was never seen again. On True Blood, the show based on the books, she was the Big Bad of season two.
Similarly, Lafayette Reynolds has been a pretty prominent supporting character since the show began, but appeared only briefly in book one before being killed off at the beginning of book two.
Tara, Lafayette's cousin, wasn't even mentioned in the first novel, but she is Sookie's best friend through the whole series.
Paul from Cheers. But not as much as Frasier, who went from just another bar patron to a main character, to the star of his own Spin-Off.
Actually, Frasier was a guest star who was only intended to be a Romantic False Lead who would vanish after one season. He was so popular, he stayed for nine seasons on Cheers and did another eleven on Frasier.
Becker in Primeval. Originally just a named redshirt. Now a main character with, apparently, his own love interest.
Gunther in Friends. He was originally an extra appearing in the background of a few Central Perk scenes. The show's executives decided to give him a named role in Season 2 because he was the only extra who actually knew how to work the coffee machine. He became a bigger regular after Season 3, when his character was given a hopeless crush on Rachel as a Running Gag that would continue for the rest of the series.
Benjamin Linus from Lost was only supposed to appear in three episodes in season two, but the writers liked him so much that they rewrote him to become a major character in the series.
The exact same thing happened to Desmond, a three-episode guest star in season two who became an important regular in the subsequent four seasons.
The writers tried to create an illusion of Ascended Extra for Nikki and Paolo by pretending that they were "there the whole time, but weren't being focused on" when they were introduced in season three.
Once Upon a Time has Belle who only appeared in three episodes for the first season and was upgraded to a main character in the second season.
Killian Jones, or Captain Hook as he's better known, was originally supposed to be a minor recurring role appearing in the first few episodes of the show's second season, but the producers and crew loved Colin O'Donoghue's performance so much that he was promoted to a member of the main cast midway through it, before his first episode even aired on television. Come Season 3 and onward and Killian has become one of the show's central characters.
The Zyu 2 monsters for the first and second seasons of Mighty Morphin' Power Rangers are kind of this trope. The Super Sentai footage was exhausted by "Doomsday," so Saban contacted Toei to produce more action footage to keep the season/series going, and the Zyu2 episodes were shot with monsters and battle footage exclusively for the US series. Starting with Goo Fish and finishing with Turban Shell, each of them started as a Monster of the Week like any other. Since they were US-only monsters, Saban owned the costumes exclusively without having to rent them. As Saban had to fill out the 50-episode second season, bringing back hordes of old monsters became a fairly regular occurrence as the season wore on, and most (if not all) of those monsters would be from the Zyu 2 crowd. Some even showed up for the grand season 6 finale at the end of Power Rangers in Space, and even the monster hell scenes from the final arc of Power Rangers Lightspeed Rescue, (season eight.) Of course, these scenes contained many monsters from across franchise history, American-original and Japanese-original.
Season two had Zedd summon some past monsters to protect the Rock of Time... immediately followed by past monsters being involved in the wedding of Rita and Zedd. Some of them were the same ones. This meant we had one-shot monsters from ages past returning for four episodes in a row. Dramole in particular totals six appearances in which he actually does something (not crowd filler like in Countdown.) Undoubtedly a record for mostToku.
The Ultra Series has many iconic recurring monsters. But without a doubt, the biggest one of these Breakout Villains is Gomora, the first monster in the franchise to defeat Ultraman. He's appeared in every major entry since 2005 and has also been a hero of a few of them since 2007(unlike the many other monsters who've reappeared frequently), fighting many villains and numerous other monsters alongside the Ultramen and defense teams as well as marking the franchise's first Heel–Face Turn from a Monster of the Week.
Kamen Rider Femme was an extremely minor character in Kamen Rider Ryuki, as she only ever appeared in The Movie (not even showing up in the series proper), and was killed off rather unceremoniously. Kamen Rider Siren, her American counterpart from Kamen Rider Dragon Knight, was a pretty prominent character, and showed up in 16 episodes.
The British TV Show Skins has a great example in Effy, who was the little sister of her main character Tony for series 1 + 2, but appeared in only a few episodes and never spoke. When the new cast was picked for series 3+4, Effy was promoted to a full character and became the main protagonist. Effy remains the only character to be involved in more than three series of skins. (Pandora briefly appears in Series 2, before being promoted to full cast in 3)
Zack Allen of Babylon 5 didn't even rate a name in his first several appearances, being just another faceless security guard. By Season 4, he had been promoted to the opening credits.
Archie Kennedy, as well as Styles and Matthews, from the Horatio Hornblower mini-series. The latter two characters were mentioned in the first book briefly, but given no real importance, and in the series they're promoted to Those Two Guys and prominent lower-deck characters. Archie is somewhere between this and Composite Character, as he is mentioned by last name in the first book, but his personality is made up of fragments of other minor characters from the books. Still, he goes from being a midshipman who appears in one scene to being the title character's best friend — a definite step up.
Richardson in Deadwood. The actor was hired as an extra and eventually placed as Farnum's cook. The writers gave him a few incidental lines and liked the results, so he was expanded to a fully-fledged supporting character.
On Boy Meets World, Angela first appears in season 5 episode 2 as a student in Feeney's class who has one line of dialogue, and then in season 5 episode 7 she has a much larger role as Shawn's girlfriend and from there she becomes a recurring character and then a main character.
Several initially-unimportant characters in The Wire gain greater prominence as the series goes on:
Probably the most notable example is Kenard, who is introduced in a very short scene from the third season where Bunk Moreland sees him and two other children arguing over who will pretend to be Omar. In the fourth season, he gets more dialogue and scenes with the neighbourhood children, and in season 5, he is the one who shoots and kills Omar in a convenience store.
Detective Colicchio was initially an unnamed background character in Major Colvin's unit before gaining an abrasive personality and more dialogue.
Jeff Price (a court reporter for the Baltimore Sun) appears in one season three sequence (a press conference) asking a question, then becomes a full-fledged supporting character in the fifth and final season.
A young woman is seen as an extra during one sequence in Hamsterdam from the third season, has a couple lines of dialogue during a scene with Old-Face Andre in season 4, then is shown at a recovering addicts' meeting Bubbles is attending in season 5.
Ronnie Gardocki from The Shield. The character was initially treated as a glorified extra, and the only reason why actor David Rees Snell took the part was because creator Shawn Ryan needed someone to fill out the team. You could count the number of lines Ronnie had in the first season on one hand. Yet, as the seasons continue, Gardocki becomes more and more central to the power struggle between Vic and Shane, and becomes a fully-developed main character in his own right.
Both Al and Heidi on Home Improvement. Al was originally a temporary character because the producers were waiting out for another actor to play Tim's assistant on Tool Time, but that didn't go through and they made Al a permanent character. Heidi's role on Tool Time was expanded from just introducing Tim and Al and helping bring out tools to having a part in the projects on the show, and she was given more screentime outside of Tool Time which included a few storylines about her personal life.
Community has quite a few example of one time characters being brought back for later episodes. Leonard is a pretty prominent example.
Mystery Science Theater 3000 - Pearl Forrester was initially a very minor character who showed up occasionally for some jokes at her long-suffering son Clayton's expense. When Frank left the show before the abbreviated seventh season, Pearl moved into Deep 13 permanently, though she was still used mainly for mother jokes and wasn't really a villain. Then the show was resurrected sans Clayton and Pearl became main antagonist for the remaining three-year run.
Sir Leon from Merlin was originally just an extra who was chosen to speak on behalf of the Redshirt Army of Camelot knights. He proved so popular with both the cast and the audience that the writers brought him back after he was seemingly killed off, and upgraded him to Arthur's second-in-command.
Kari Byron made her first on-camera appearance in a pilot episode of MythBusters, for the vacuum toilet myth. (She had her rear end scanned.) She reappeared, alongside fellow Build Team members Scottie Chapman and Tory Belleci, at the beginning of the second season.
Jimmy Palmer of NCIS started in the middle of the first season as a replacement for largely identical character. In Season 6 he was promoted to "Also Starring" and in Season 10 he is promoted to a series regular.
JAG: Petty Officer Jason Tiner (played by Chuck Carrington), the Admiral's yeoman, went from an extra and became a minor character in his own right after a few seasons, and apart from the lead characters, appeared in the highest number of episodes.
Supernatural: The final episode of the first season introduced Bobby Singer, a hunter who'd worked with Sam and Dean in the past. He shows up at the very beginning to help them out against Meg and then disappears for the rest of the episode. Come season two, Bobby's back aiding the boys after their father's death and goes on to become a father figure to them who's pretty much the most important character on the show next to the boys.
When Castiel was introduced in Season 4, he was only meant to last a few episodes. But due to positive fan reaction, he remained as a recurring character in Season 4, was upgraded to a regular in Seasons 5 and 6, downgraded to recurring for Seasons 7 and 8, and has been announced as a regular for Season 9.
Crowley appeared in the fifth season, one that was originally intended to be the final one. He first appears briefly in the middle of it, then doesn't show up again until the tail end of it, both times providing some aid for the boys against the much worse antagonist Lucifer. Then the show continues going after that but there's one problem: now that your Big Bad for the entire series up to that point is out of the picture, who do have take his place? That's when Crowly returned front and center and went on to become the show's primary antagonist, even getting promoted to a regular for Season 10.
In the first season, Glenn Morshower played Agent Aaron Pierce, a member of David Palmer's security detail. He originally had very few lines, and mostly stood around in the background of shots while Palmer did campaign speeches. However, his importance grew in subsequent seasons - in the second, he has a plot arc where he betrays the President and subsequently apologizes. The production crew were planning to kill him off in season 5, until Morshower's performance in a late-season episode impressed them so much that they hastily changed the script to keep him alive. He would then go on to have further appearances (and even helps Jack during the siege on the White House in season 7), and is one of only 4 characters from the first season to have survived through the entire series.
The sixth season introduced a relatively minor character named Ethan Kanin who was the Secretary of Defense. He briefly shows up a couple times to talk about a few decisions regarding the current President and overall was pretty forgettable. Then Redemption and the last two seasons come in and he becomes the Chief of Staff under the new President often acting as her only real confidante during some of the harsher moments on the show.
The Office (US): A lot of characters qualify as this. Basically everyone except Michael, Dwight, Jim, Pam, and Ryan. In the first season, much of the supporting cast were basically extras who didn't have any storylines or personality traits until season 2.
The Price Is Right has always had the game show staples of Lovely Assistant (a rotating cast of models) and The Announcer. But early on, the models just showed off prizes, and the announcer just read copy. After a few years, both the models and announcer began interacting with host Bob Barker a lot more, usually conversing with him between segments, being shown on-camera more, and even participating in humorous skits during the Showcases at the end of the show. The interaction died down some in the 1990s and 2000s, but came back in the late 2000s after Drew Carey took over as host.
Similar to the Friends example, in 7 Vidas Gonzalo de Castro just played the waiter at the bar the main characters went to, but soon became a main character and stayed in the series until the last chapter.
In The Sopranos, Adriana La Cerva, the girlfriend of Christopher Moltisanti, originally appeared in the series pilot as an unnamed hostess at Artie Bucco's restaurant with just a couple of lines. However, producers of the show were so impressed by what they saw of Drea De Matteo's acting skills, that she was quickly promoted as Christopher's love interest and a major character in the show.
On the 1970s Beat the Clock revival, announcer Gene Wood was the celebrity partner for a week, with Nick Holenreich taking Wood's usual role. After Jack Narz stepped down from hosting in 1972, Wood was promoted to the hosting position, and Holenreich became the permanent announcer.
Promptly inverted on the 1980 revival, which had Narz as announcer and Monty Hall as host.
Tobias Gregson was an extremely minor character in the original Sherlock Holmes stories, but Captain Gregson is a major character in Elementary. He essentially fills the role usually held by Lestrade in the original stories and most adaptations.
Ruthie Cohen, played by extra Ruth Cohen, on Seinfeld—the cashier at Monk's Cafe. Appeared in 101 episodes of the series, more than twice as many as Newman! Ascended to actually speaking lines in two episodes, and was a central character for one. Also appeared as the cafeteria cashier on NCIS, referencing her Seinfeld role.
Harshly subverted with Babu Bhatt, who in his second appearance shows every sign of being promoted to a recurring character by working at Monk's and living in Jerry's building. Then he's deported. In the DVD special features, Brian George recounts his experience reading the script, and the sharp emotion switch upon realizing this would be it for him.
Aída started as Sole's cleaning lady in Siete Vidas. Her character was so interesting she was soon made a regular and given a job at the local pub. She eventually got her own spin-off, Aída which survived the main series for over a decade (and, as of July 2013, is still going).
Sukar was suppossed to be a one off character in Defiance. However, the creators were impressed by the contributions that Noah Danby made in defining the Irathient race that he was was written into more of the stories, and eventually ascended to a Sacrificial Lamb.
Detective Tierney had bit parts in a number of episodes throughout seasons 1 and 2, mostly as just another face around the 8th Precinct, but the final scene from "All In" paved the way for him to become a more significant character.
A character, played by Martin Clunes appeared in a film Saving Grace and later two prequel miniseries. In making Doc Martin, the character was given a Re Tool with Doctor Jerk added to the character (who was originally just a Fish out of Water), and his last name was changed to Ellingham (an anagram of the last name of the show's writer Dominic Minghella).
On Degrassi: The Next Generation, some minor/recurring characters have been upgraded to regular cast as some of the characters get older and graduate. For example, Clare was originally introduced as a minor character, the middle-school aged sister of one of the main characters who appeared only a couple of times. She became a main character beginning with season 9-10.
Early Degrassi has no proper examples of this, as the kids were pretty much always playing their parts whenever they were onscreen. It wasn't uncommon to see Snake in the background while a scene focused on Melanie and Kathleen.
Max Bergman was originally just a one-episode test character on Hawaii Five-0. He proved likeable enough that he now shows up about Once an Episode, usually to autopsy a body but also to celebrate with the gang after a case. Being played by Masi Oka of Heroes fame probably helped.
Arrow: Felicity Smoak may well be the crowning example. She started as a one-scene character, never intended to return, but her portrayal by Emily Bett Rickards was well-received by both the creators and the audience and she was promoted to a recurring role in early-mid season 1. By season 2, she was a main character, and coming into season 3 was essentially the female lead of the show.
Bellamy Young was essentially bound to play a token First Lady who would be unseen and eventually pushed off to the side in the first season of Scandal, but Mellie Grant's rivalry with Olivia Pope and Young's strong interpretation of the role eventually bumped her up to regular status and kept her in the White House.
Donna Moss (Janel Maloney) of The West Wing wasn't an extra, rather one of the staff assistants who appeared consistently but with a minor role. Her chemistry with her boss Josh quickly overshadowed his intendedFoil relationship with main cast member Mandy, along with their UST. Come season two, Mandy went to Mandyville and Janel Maloney was in the opening titles, ending the series as the new First Lady's Chief of Staff.
Maxwell Klinger was initially meant to be a one-off character on M*A*S*H, a corporal who took to crossdressing in an attempt to get a Section 8 psychiatric discharge from the army. (Little-known fact: the original script for the episode in which he appeared had him as a highly effeminate gay man, but the writers decided it would be "more interesting" to have his crossdressing be an attempt at getting a Section 8). The character was so well-received that they made him a regular, adding to his list of antics in his attempts to get discharged, and by the end of the show he was one of only four characters to appear in all eleven seasons.
Doremy Vernon first appeared as the surly canteen manageress in the first three Christmas episodes of Are You Being Served? By 1979, she had ascended to a semi-regular character on the show.
All My Children: Erica Kane could be consider a staple of this trope. When first introduced she was a recurring Antagonistic Foil, the spoiled diva daughter of the saintly maid Mona Kane, to young Ingénue Tara Martin, the classic soap heroin. However she quickly stuck a chord with audiences, and in the end Tara was Put on a Bus and Erica would become not only the show's central Hero or Anti-Hero depending on the writer, being the longest lasting character and the only one to be a part of the show for the entire 41 year run, but also the most Iconic and Famous Character in Daytime Television.
The Games: Has Nicholas the Secretary to the Minister for the Games promoted from minor role to main cast member. Also Tim the secretary grows in prominence throughout the second series.
Colin Fassnidge, a guest judge introduced in the fourth season of My Kitchen Rules was given the role of the main judge in Season 6, where he replaces Manu as the co-judge of the People's Choice Challenges.
Chiyoh/Chiyo is a much more important character in the Hannibal TV series than she was in the novels, where she was just a young girl working as a handmaiden, and only ever appeared in one book. This is largely a case of Real Life Writes the Plot; Bryan Fuller wanted to work with Tao Okamoto after seeing her in The Wolverine, and decided to cast her as Lady Murasaki, an important character from Hannibal's backstory. However, he became concerned that Tao was too young to be believable as Mads Mikkelsen's aunt, so he rewrote the role to be Chiyoh, who was essentially the only other Japanese female of any note in the franchise.
The character of Celeste Dexter in Night and Day has a walk-on role as one of Sam Armstrong’s many conquests very early in the run, then returns nine months later (and not even pregnant!) as a prominent character, remaining in the show until the end of the series.
Most of the characters in Agents of S.H.I.E.L.D., by necessity. Because almost any character with any real prominence in the comics has been earmarked for use in the MCU films or Netflix shows, AOS largely has to deal with obscure barrel-scrapers who never got much page time in the comics. For instance, despite only ever appearing in just over a dozen or so comic issues before the show, Lance Hunter is one of the main characters.
In the miniseries adaption of 11/22/63, the character Bill Turcotte has a much larger role than he had in the book. In the book, he only appears during the part set in Derry. In the series, he becomes Jake’s ally in his quest to save Kennedy, and moves with him to Jody, posing as Jake’s little brother.