Also Peter Capaldi.
Went from a strong second banana in Doctor Who’s Fires of Pompeii (David Tennant’s Doctor) to being THE Doctor two Doctors later (you know what I mean). It was strongly enough remarked upon that they came up with an explanation for it.
I once heard Backus tell of being in a nudie flick (apparently a big-budget production) back in the 1930s.
As for Nielsen, he did straight drama for most of his career, often appearing as the heavy. He played studio head John Bracken on Bracken’s World (1969–70) and was a CIA operative in an episode of Columbo.
Does a hot dog make him lose control? :dubious:
Daniel J. Travanti had a bit part in an episode of the original Lost In Space (credited as “Danny Travanty,” IIRC).
Gavin MacLeod and William Christopher (before he was Father Mulcahy on MASH* - well, the second Father Mulcahy, but that’s another story) played Germans (and possibly an Englishman or two) in various episodes of Hogan’s Heroes. Jack Riley (Mr. Carlin on The Bob Newhart Show; Stu Pickles on Rugrats) played a German (SS?) soldier in one as well.
Alan Alda and Julie Newmar both played bit parts on episodes of The Phil Silvers Show.
Between which he played a doctor with the World Health Organization in World War Z. A W.H.O. Doctor.
In Christopher Eccleston’s Jude, David Tennant has a walk on bit as “drunken undergraduate”.
John Ritter had a reoccurring role as Reverend Matthew Fordwick on the Waltons before hitting it huge on Three’s Company.
MacLeod was also a regular on “McHale’s Navy”.
I once won gift certificates to a restaurant by stumping a DJ with the trivia question, “On which ship did Gavin MacLeod serve before captaining ‘The Love Boat’?”
Diana Muldaur played two different characters in different episodes of Star Trek: TOS before returning to play Dr. Pulaski in STNG.
He was also the tennis-playing minister who married Ted and Georgette on The Mary Tyler Moore Show.
And just a few years later, Smith played the grownup version of the narrator/protagonist in the movie Auntie Mame with Rosalind Russell.
In a similar vein, Det. Wentworth was set to become Wojo’s girlfriend until Linda Lavin landed Alice.
He was also Rev Chatfield, the tennis-playing priest who married Ted and Georgette at Mary’s new apartment on MTM.
EDIT: Damn, ninja’d! 
Of course, technically speaking, the PT 73 was a boat, and the Pacific Princess was a ship.
In general, a ship can carry a boat, but a boat cannot carry a ship. 
Colm Meaney, who played Miles O’Brien on Star Trek: The Next Generation and Deep Space Nine, had a very minor role in Dick Tracy as a uniformed police officer. If I recall correctly, it was a speaking role, but he said all of two or three words.
Christopher filled in for Sgt Andrew Carter in an episode of Hogan’s Heroes when Larry Hovis, for some reason, wasn’t available. I used to think he actually assumed the role of Carter, but upon watching that episode I saw I was wrong. His character’s name was Thomas.
Even more gobsmacking is seeing him in his New Wave / New Romantic band. Just do a Google Image search for Ricky Gervais Seona Dancing. You won’t be disappointed.
Back on topic; a few weeks ago I saw George Clooney in an episode of Murder, She Wrote.
He was hilarious as the head of an Irish household in The Commitments. I laughed out loud when I saw the portraits of Elvis and the Pope hanging side-by-side on his living room wall!
He was also pilot of the doomed British airliner in Die Hard II.
In the 1950s, Pert Kelton played Alice Kramden on The Honeymooners until she was blacklisted and replaced with Audrey Meadows. In the '60s, she played the mother of Sheila MacRae’s Alice in Jackie Gleason’s revival of The Honeymooners.
Trixie, the wife of Ed Norton on The Honeymooners, was played in one episode by Elaine Stritch. She was replaced with Joyce Randolph.