Just about every tv show has those characters that were supposed to be one-offs or only last a couple of episodes as reoccurring characters. Here is a look at some characters that managed to make it from part-time to full-time and how they changed their respective shows.
Felicity SmoakPlayed By: Emily Bett Rickards
-Felicity was supposed to just be an IT girl that helped Oliver/The Hood on 1 or 2 missions. Emily Bett Rickards impressed the studio and audience so much that they kept her around the entire season and promoted her to full time at the beginning of their 2nd season.
-Bringing in Felicity full time changed the series for the better or the worse depending on who you ask. She went from a part time character to a character that rivaled the main character not only in screen time but also in popularity and would end up to take over the main role of love interest.
Spike 'Buffy The Vampire Slayer'Played By: James Marsters
-Spike aka William the Bloody appeared in BTVS Season 2 as the big bad who wanted to kill The Slayer. Spike was supposed to meet his demise in the middle of the season in order to allow his girlfriend, Drusilla take up the mantle of big bad alongside Angelus. Due to James Marsters badass portrayal of the character and reception with the audience, Spike survived the season and became a regular in Season 4.
-Spike would go from big bad to full time Scooby member and would eventually become Buffys new love interest for the shows last 2 seasons. Not even death could keep the character away as he jumped ship after BTVS ended and became a series regular on the sister show, Angel for its final season.
Santana Lopez & Brittany S. Pierce 'Glee'Played By: Naya Rivera and Heather Morris
- Santana Lopez and Brittany S. Pierce (also known as Brittana) started out as simple lackeys and backup dancers to Quinn Fabray. Half way through the first season they became fan favorites breaking out from under Quinn's shadow. By season 2 both were promoted to series regulars.
-Due to their popularity both as individuals and together, they would go on to get several solo/team up performances along with becoming a power couple on the show, culminating in their wedding during the shows final season.
Blaine Warbler 'Glee'Played By: Darren Criss
- Blaine Anderson was introduced in Glee's 2nd season as a mentor and possible love interest to Kurt Hummel. He was quickly promoted to a series regular at the beginning of the shows 3rd season.
-Blaine was a stand out character to teenage girls around the world very quickly with his rendition of Katy Perrys 'Teenage Dream', a song that would go on to be one of the shows best selling singles. He quickly overshadowed the shows original muse, Kurt Hummel. The two would go on to become the shows most popular couple.
Sophia Petrillo 'Golden Girls'Played By: Estelle Getty
-Sophia went from a guest character in the pilot of Golden Girls to a series regular by the second episode.
-Sophia was the wise cracking mother that was never afraid to call it like she saw it which helped make her a popular character. She would go on to out last all the shows characters by appearing in various spin offs such as The Golden Palace and Empty Nest.
Harmony Kendall 'Angel'Played By: Mercedes McNab
-Harmony is a character that truly stood the test of time. She started out on Buffy The Vampire Slayer as a reoccurring character throughout the shows 7 seasons before becoming a series regular on the spin off, Angel, in it's 5th and final season.
-Harmony went from being Cordelia Chase's lackey to Spikes girlfriend to Angels assistant. She was a dumb blonde both in her human life as well as her vampire life but was always happy go lucky which endeared her to the audience. She didn't make any big splashes though.
Angel "BTVS/Angel'Played By: David Boreanaz
- Buffys steamy vampire beau, Angel was not originally meant to last past S1 but his chemistry with SMG and general charisma made the crew/fans fall in love with him. He was officially upped to a regular in S2.
-Angel would go on to be such a popular character that he went on to get his own successful spin off that lasted for 5 seasons. He went from playing love interest to Buffy to playing the main big bad in season 2 to breaking Buffys heart in S3.
Source: Me, myself, and I (avid watcher of all the shows listed)
I know I probably missed plenty so get to listen some of your faves that were promoted from guests to full time and if it was a good decision