Jesse Pinkman (Aaron Paul), “Breaking Bad” — Series creator Vince Gilligan had originally planned to kill off Jesse Pinkman at the end of the first season in what was supposed to be a botched drug deal as a plot device to plague Walter White with guilt. Gilligan made him a series regular, however, after Paul demonstrated a strong ability to act and his character proved to be so popular with audiences.
Boyd Crowder (Walton Goggins), “Justified” — Boyd was supposed to die in the pilot episode when Raylan and Ava shot him. However, he was so popular with test audiences that he was written in as a recurring character. By the second season, he was elevated to a series regular.
Andy Bernard (Ed Helms), “The Office” — Originally signed on for only 10 episodes, Bernard shared so many similarities with a character the producers had already planned on creating that they simply merged the two and brought Andy back as a series regular. He’s now the manager of Dunder Mifflin. Sadly.
Sheldon Cooper (Jim Parsons), “Big Bang Theory” — Originally written as a minor character — the roommate and colleague of the show’s main character — his popularity with audiences resulted in his character being elevated to one of the featured characters. His now the biggest reason for the success of the show (besides the laugh track).
Benjamin Linus (Michael Emerson), “Lost” — He was only supposed to be on the show for three episodes, but the producers liked him so much that they elevated him to the leader of “The Others” and later made him a series regular.
Sophia Petrillo (Estelle Getty), “The Golden Girls” — She was only supposed to appear in the pilot, but she proved so popular with audiences that she was elevated to series regular, replacing a gay chef named “Coco” who was supposed to be the fourth member of the regular cast.
Steve Urkel (Jaleel White), “Family Matters” — Originally, Urkel was supposed to appear in only one episode as a nerdy kid who dated Laura Winslow. He proved so popular that the producers brought him back for several more episodes, and he was so winning he was elevated to series regular. Now, he is who most associate with the show.
Mimi Bobeck (Kathy Kinney) “Drew Carey Show” — She was only supposed to appear in the pilot episode, but was such a hit with producers that she was written into the show permanently.
President Bartlett (Martin Sheen), “West Wing” — Originally, the president wasn’t even supposed to be in “The West Wing,” Aaron Sorkin had meant for the focus to be entirely on the senior staff. However, he decided to put a face to the President’s name, signing Martin Sheen on for only four episodes. So impressed with his performance, Sorkin wrote him in as a series regular.
Dean Pelton (Jim Rash), “Community” — Several members of the “Community” cast could be considered for this list, as the show was meant to focus mainly on Jeff Winger. However, it soon became an ensemble show, and early scene stealers Senor Chang and Dean Pelton were elevated to series regulars in the second and third seasons, respectively.
Sue Sylvester (Jane Lynch), “Glee” — Written only as a recurring character, she was elevated to series regular after a commitment to another television show (a Damon Wayans pilot) was not picked up.
Frasier (Kelsey Grammar), “Cheers” — As originally written, Frasier was only supposed to appear on a few episodes of “Cheers,” as the love interest of Diane Chambers and a brief rival to Sam Malone. However, after Diane left him at the altar, Frasier began coming into the bar regularly, became friends with Sam, and eventually rose to a series regular (and would later, of course, land his own spin-off).