How about Warren Beatty as Milton Armitage in the first season
of "Dobie Gillis"? Likewise, Michael J. Pollard (pre-"Bonnie And
Clyde") as Barney's cousin Virgil on "The Andy Griffith Show."
Phil Silvers had a couple of then-unknowns on the Bilko show;
Dick Van Dyke played a Southern mountain boy with a wicked
baseball throw; naturally, Bilko tried to get a major-league team
to sign him. Alan Alda made a brief appearance on another episode,
but I'm not sure what he played.
And while I wouldn't classify Aaron Spelling as a star, just one of
the mega-producers of all time, he convinced Lucy (through Gary
Morton) to do that disastrous "Life With Lucy" in 1986. On "I Love
Lucy" he appeared in the episode where the Ricardos and Mertzes
were arrested for speeding in Bent Fork, TN, and Lucy's Cousin Ernie
(Tennessee Ernie Ford) had to bail them out. Spelling was the loopy
gas station attendant who couldn't quite tell Lucy, who was driving,
how to get back on the main road to Nashville.
Then there was Greg Morris, pre-"Mission: Impossible," who appeared
on the Van Dyke show to convince Rob Petrie that their babies had not
been switched at birth.