20 TV actors who became series regulars after slaying their performances

This is arguably the most legendary example in TV history. Steve Urkel was meant to be a one-off nerdy neighbor, but Jaleel White's hilarious and heartfelt performance was so popular with audiences that he became the undeniable star of the entire show.

Perhaps the show's most impactful addition, Joe Morton, was introduced as the mysterious head of B613, Rowan Pope. His commanding presence and delivery of epic, world-stopping monologues made him an instant legend. Morton's promotion felt less like a casting decision and more like a force of nature that the show wisely decided to embrace.

Dr. Frasier Crane was only supposed to be Diane's snooty, temporary love interest for a handful of episodes. However, Grammer was so brilliant that he not only became a main fixture at the bar for nine seasons but also landed an 11-season spinoff that became a classic in its own right.

The sleazy lawyer Saul Goodman was originally written for a four-episode arc to solve a plot problem. Bob Odenkirk's performance was so wildly entertaining, though, that he became a series staple and, eventually, the star of his own critically acclaimed prequel, Better Call Saul.

Initially introduced as the meek and mysterious "Henry Gale," Michael Emerson was so brilliant that the creators expanded his role. He became the show's primary antagonist, the manipulative Ben Linus, and one of TV's all-time greatest villains.

The angel Castiel was only supposed to appear in a few episodes at the start of season four. Fans immediately fell in love with Misha Collins' awkward and endearing portrayal, making him an essential third lead alongside Sam and Dean for the rest of the show's epic run.

The punk-rock vampire Spike was designed to be a short-term villain who would be killed off after a few episodes. Marsters had such incredible charisma and chemistry with the cast that he not only survived but evolved into a complex anti-hero, love interest, and a core member of the Scooby Gang.

Daryl Dixon was created specifically for Norman Reedus after he auditioned for another role. His magnetic performance as the tough-but-tender crossbowman made him a fan favorite and the eventual face of the franchise.

Ben Wyatt was introduced in Season 2 of Parks and Rec as a cold-state auditor sent to slash Pawnee's budget. Adam Scott's fantastic comedic timing and chemistry with Amy Poehler quickly made him an undeniable part of the cast.

First Lady Mellie Grant was initially a recurring character, often seen as an obstacle in the show's central romance. Bellamy Young brought such depth, humor, and fire to the role that she was promoted, eventually evolving Mellie from a scorned wife into a formidable political powerhouse in her own right.

Steve Harrington was written as the stereotypical, unlikable '80s jock, intended to be a minor antagonist. Joe Keery's performance was so charming that the Duffer Brothers rewrote his character, transforming him into a beloved, bat-wielding babysitter and a hero of the series.

Mike Ehrmantraut was literally a character born out of scheduling conflicts, written into an episode when Bob Odenkirk was unavailable. However, Jonathan Banks' intensity made such an impact that Mike became a cornerstone of both Breaking Bad and Better Call Saul.

Dr. Addison Montgomery was brought in for a short arc at the end of season one to create a dramatic love triangle. Kate Walsh's performance was so commanding and popular that she became a series regular and later starred in her own successful spinoff, Private Practice.

Dr. Callie Torres was introduced as a recurring love interest in season two. Sara Ramirez's powerful performance resonated so deeply that Callie became a central character for a decade and one of the longest-running LGBTQ+ characters in television history.

Introduced as the mysterious B613 agent Jake Ballard, Scott Foley was brought in for a recurring arc to complicate Olivia's life. His chemistry with the cast and the character's integral role in the show's mythology made him a fan favorite and a permanent fixture in the main love triangle.

Mike Hannigan was just supposed to be a minor character Phoebe was set up with on a blind date. Paul Rudd's effortless charm made him an instant hit, though, and he became a fixture in the final two seasons as Phoebe's eventual husband.

The character of Erin Hannon was added in season five as a temporary receptionist. Ellie Kemper's sweet, quirky, and hilarious performance made her an immediate fan favorite and a core member of the Dunder Mifflin family until the very end.

The brilliant and eccentric political operative Eli Gold was introduced as a guest character in the first season. Alan Cumming's scene-stealing energy was so captivating that he was brought on as a regular, becoming one of the show's most beloved and essential characters.

Dr. Kerry Weaver was introduced as a recurring physician in season two to consult on a few cases. Laura Innes's complex portrayal of the tough, abrasive, and ultimately brilliant doctor, however, made her an integral part of County General Hospital for the next 12 years.

Portia de Rossi arrived in season four as the ruthless RNC chairwoman Elizabeth North, a recurring political shark. She was so effective as a cunning rival that she was promoted the following season, becoming a major D.C. power player and a fantastic addition to the main cast.