Cast members who appeared in Third Rock from the Sun (aka 3rd Rock) included leading and supporting actors together with an impressive array of guest stars.
A few of those who had supporting roles appeared in the opening episodes and remained throughout the show's run. Others were introduced during the course of the series but had established themselves in roles as regulars by the end. Some of the show's guest stars, like William Shatner, John Cleese and Jan Hooks, also had recurring roles.
According to the Internet Movie Database only John Lithgow, Kristen Johnston and French Stewart appeared in all 139 episodes.
By Season Six Joseph Gordon-Levitt seems to have been pursuing outside interests and appeared irregularly. It was written into the storyline that his character as Tommy enrols at university which, though located close enough to allow him to return occasionally, meant having to move out of the Solomon household. Tommy remained part of 'the unit' but only appeared in about half of the episodes for the final season - 133 episodes in total.
Otherwise (if the figures are to be believed) Jane Curtin appeared in 125 episodes, Simbi Khali in 112 episodes; Elmarie Wendel in 83 episodes and Wayne Knight in 77 episodes.
John Lithgow played the role of Dick Solomon, High Commander and leader of the expedition that also includes Sally (played by Kristen Johnston) who is his Lieutenant and second-in-command. On the same mission are& Tommy (Joseph Gordon-Levitt), a seasoned intelligence operative whose role is that of Information Officer, and Harry who is played by French Stewart.
In order to carry out their mission, they each assume human roles and collectively try to pass as an 'average American family.' For this purpose Dick takes a job as professor of physics at Pendleton University, where he shares an office with his on-again, off-again lover, Mary Albright, played by Jane Curtin.
They also share their office, much of their professional life and some of their personal anxieties with a secretary by the name of Nina Campbell, who is played by Simbi Khali.
In order to have a base of operations from which to study intelligent life on Earth, they rent an apartment from Mrs Dubcek, played by Elmarie Wendel. She has a daughter named Vicki, who was played by the late Jan Hooks, in a recurring role.
Wayne Knight played the role of Officer Don Orville, authority figure and Sally's principal love interest. Introduced initially as a recurring character, he emerged as a series regular by the end of the show's run.
Two recurring guest stars appeared in alien roles. William Shatner played the Big Giant Head (who else?) while John Cleese played the pompous and overbearing Dr Liam Neesam, which surely must have been a challenging role for him!