A one-shot appearance in Season 1 as Reverend Jim Ignatowski (to perform a “marriage of convenience” between Latka and a prostitute, so Latka could stay in the U.S.) became another appearance in Season 2, and a regular role by later in that season.
Kristen Johnston, Mom
Her appearance as Tammy Diffendorf on season 5 was meant to be a one-off, but viewer response was hugely positive, so she appeared regularly beginning the 4th episode of season 6, and was in the opening montage by season 7.
Actors whose career never took off after their TV show ended
Soleil Moon Frye, Punky Brewster
George Reeves, Superman
Dustin Diamond, Saved by the Bell
Played the Jughead archetype, Screech, on saved by the Bell and a bunch of spinoffs, but after that was pretty much reduced to playing himself in guest appearances on random shows.
Actors whose career never took off after their TV show ended
Soleil Moon Frye, Punky Brewster
George Reeves, Superman
Dustin Diamond, Saved by the Bell
Matthew Fox, Lost
Chevy Chase, Community
Jenna Elfman, Dharma & Greg
Erik Per Sullivan, Malcolm in the Middle
One of my all-time favorite TV characters (Dewey), both in the writing and performance. He did some work other than Malcolm, but nothing recent as far as I know, possibly by his own choice rather than lack of interest from the industry.
Actors whose career never took off after their TV show ended
Soleil Moon Frye, Punky Brewster
George Reeves, Superman
Dustin Diamond, Saved by the Bell
Matthew Fox, Lost
Chevy Chase, Community
Jenna Elfman, Dharma & Greg
Erik Per Sullivan, Malcolm in the Middle
Michael Richards, Seinfeld
Gates McFadden, Star Trek: The Next Generation
She had a starring role on one show that lasted 13 episodes; a few one-off guest roles; she was in the four TNG films; and the third season of Picard, and that’s it.