Roger Smith of 77 Sunset Strip, about 16 years old in the Wagon Train episode, The Dick Pederson Story.
George Wendt, NORM!!! of Cheers, as a Marine with a pool ball in his mouth, in an episode of MAS*H.
Christina Applegate (Kelly Bundy) in an episode of Family Ties when the younger daughter joined a band.
Pre-SNL Adam Sandler on The Cosby Show as one of Theo’s friends in 2-3 episodes
more interesting is an actor who plays a minor role on a show and then becomes a main character on that show in later episodes. For example the guy who played Vito Spatafore on the Sopranos was in an earlier episode playing a customer in a store.
In the first season of The Avengers, David Keel (played by Ian Hendry) was the lead, and John Steed (played by Patrick MacNee) was the sidekick. When Hendry declined to do a sequel, they made John Steed the lead character.
Back to the OP:
John Ratzenberger (Cliff Claven in Cheers) as Bill, the Electrician in House II: the Second Story.
Also appeared in Superman, Superman II, The Empire Strikes Back, Outland, and Gandhi.
Krull and Excalibur had a lot of British actors who would later become famous.
And then there is Patrick Stewart’s infamous role in Lifeforce.
I think the granddaddy of that has to be Harry Morgan as General Bartford Hamilton Steele in an early episode of MASH then coming back to play Colonel Sherman T. Potter for the remainder of the series after McLean Stevenson left.
the other day I saw Currie Graham play a “skel/crook” on NYPD Blue and later in the last season he played the Lt./boss of the detective squad.
Happened all the time on Law & Order. Jerry Orbach was an attorney before becoming Det. Lennie Briscoe. (Greatest TV character ever, by the way.) S. Epatha Merkerson was the mother of a murdered child before becoming Lt. Van Buren. Annie Parisse was a stripper before becoming ADA Borgia. Diane Neal was a rapist on SVU before becoming ADA Novak.
Stewart was in Excalibur, too.
Dennis Franz appeared in five episodes of Hill Street Blues as crooked cop Sal Benedetto, then came back after a season to play sleazy but not evil Norman Buntz.
Jennifer Tilly played Frasier Crane’s fun-loving girlfriend Candi in Cheers, then showed up years later as Frasier Crane’s fun-loving girlfriend Kim in Frasier.
Law and Order recycled so many actors in different roles that’s it’s impossible to sort them out, but the most notable was Jerry Orbach, who played a defense attorney in one episode before settling in as Lennie Briscoe.
ETA: Dammit Robot Arm!
John McIntire played a survivor from a previous wagon train as a guest star on Wagon Train. When Ward Bond died in 1960, McIntire took on the role of wagon master Chris Hale.
I caught a rerun of Star Trek TNG the other night with Marc Alaimo playing a different Cardassian, before his long-running stint as Gul Dukat.
Kathryn Card first appeared on I Love Lucy as cleaning lady in one episode, but came back the next season in a semi-recurring role as Lucy’s daffy mom.
Gul Macet. (It’s been retconned in the novelverse to say that they were cousins. IDENTICAL COUSINS.)
Is that sort of thing common among Cardassians?
It’s hard keeping up with the Cardassians
Peter Falk, TV’s Columbo, was a Fidel Castro clone in the Twilight Zone episode “The Mirror.”
George Takei was in the Twilight Zone episode that takes place in the attic with an evil samurai sword. And Elizabeth Montgomery, as a fetching brunette, was in the TZ episode about a post-apocalyptic town where a lone female Russian (?) soldier meets a lone American soldier (who was Charles Bronson!)
Now that I think about it, Twilight Zone is rife with examples. Also from Star Trek, James Doohan, Bill Shatner, and Leonard Nimoy all had Twilight Zone appearances. Doohan was one of the townspeople in the town that had all the high tech and stayed secret; Nimoy was a soldier; Shatner was a in at least two as an airplane passenger that sees a monster on the wing and the husband where a couple is trapped by a fortune-telling machine that promises dire consequences if they leave town.
In early episodes of Hill Street Blues, David Caruso had a recurring part as the leader of an Irish street gang. Supposedly when he was cast in NYPD Blue, Stephen Bochco (who produced both shows) didn’t remember him.
You can lose your mind, when cousins are two of a kind!
Less than a year before Happy Days, Henry Winkler appeared as Rhoda’s date in an episode of The Mary Tyler Moore Show.
And Firefox.
I first knew him as Sejanus in I, Claudius.
I watched Gilligan’s Island repeatedly when I was a kid; so it was hard for me to take Rebel without a Cause seriously with Jim Backus(Thurston Howell III - the millionaire from Gilligan’s Island) playing Jim Dean’s father.
It’s also weird watching Leslie Nielsen in The Forbidden Planet after his later career turn to absurd comedy.