They're Some of the Biggest Breakout TV Characters of All Time.
I just posted a clip from this weekends Superbowl episode of Glee that features breakout character Blaine, and I explained his insanely quick rise to series regular on the show. It's strangely rare that a single character breaks out in such a big way, but here are some major examples:
Henry Winkler as Arthur Fonzarelli (aka "Fonzie"), Happy Days Orginally a secondary character, The Fonz eventually became so popular (and so integral to the action) that he became the main character on the show once Ron Howard left to direct movies.
Luke Perry as Dylan McKay, Beverly Hills, 90210 Luke Perry intially auditioned for the role of Steve Sanders, but was later cast in a recurring role as Dylan McKay. His instant popularity coupled with the show's shift in focus from family drama to teen soap turned Perry into an instant heartthrob.
Jaleel White as Steve Urkel, Family Matters Steve Urkel was originally written into a single episode, but the audience responded immediately so he was writen into another... and then made into a recurring guest star... and then a full-time cast member... and that was all in the first season. Advice to any actor doing a guest stint on a show: if they give you a line that sounds like it could become a catch phrase ("Did I do that?"), sell the HELL out of it and ride that money train to your grave.
Megan Mullally as Karen Walker, Will and Grace Karen Walker was always conceived as a series regular, but it's pretty obvious from early episodes that producers were unaware of the comic talent that Megan Mullaly possessed. In early episodes, Karen is MUCH more grounded and lacks the nasally voice that would become her trademark. Once they started to make Karen's character more broadly comedic, it broadened the prospects of the show's comedy. Now, instead of a show about 2 best friends (Will and Grace), it was a show about 4 best friends and the various combinations of them, the best being Karen and Jack).
Eddie Murphy as Various, Saturday Night Live In 1980 the final remaining members of the orginal "Not Ready for Primetime Players" left the cast, and the show's guiding force Lorne Michaels followed. In his place at the start of season 6 was producer Jean Doumanian (who's probably best known now as a longtime producing partner of Woody Allen) and a cast of new unknowns that included Joe Piscopo and Gilbert Gottfried (as well as Charles Rocket, who became notorious as the guy that got fired for dropping the show's first live F-bomb). Four episodes in, Jean hired Eddie Murphy as a featured player. The season was a disaster both critically and ratings-wise (have I mentioned Gilbert Gottfried was in the cast?), so Jean Doumanian was fired and replaced by Dick Ebersol. Dick immediately recognized Eddie's command of the screen, and told the writers to start giving him (and Joe Piscopo) as much screen time as possible.