The leading actors in the TV comedy classic Third Rock from the Sun (aka 3rd Rock) were:
John Lithgow in the role of Dick Solomon, an alien posing as a physics professor at Pendleton University in Rutherford, Ohio.
Kristen Johnston in the role of Second-in-Command, Sally who assumes the role of 'the woman' and Dick's sister for the sake of the appearances.
Joseph Gordon-Levitt, in the role of Tommy Solomon, described as 'a seasoned intelligence offider', also the oldest member of the group but posing as Dick's teenage son - mother unknown.
French Stewart played the role of Harry, not very bright but lovable all the same. His role was somewhat understated, both in terms of the storyline as well as professionally but in comedy terms he brought breadth and depth to the series.
Jane Curtin also had an important role in the series due to her character's personal and professional involvement with Dick. She played Dr Mary Albright, Dick's co-worker, on-again, off-again lover. She shared in many of the aliens adventures; was frequently dragged into their exploits and could be regarded as a central character, possibly even the fifth member of the crew.
Simbi Khali (Nina Campbell - Dick and Mary's secretary) and Elmarie Wendel (Mrs Dubcek, the Solomon's landlady) appeared in the show from the beginning and remained throughout the series. More than just supporting roles, it could be said that their characters of anchored the storyline for quite different reasons. Nina's no-nonsense take on life and in her dealings with the Solomons (and especially with Dick), with Dubcek's love of frivolity gave the aliens much to learn from as well as feed off.
Actor Wayne Knight was initially introduced as a recurring character (Officer Don Orville) but by Season 3 was a regular with an opening credit to his name. He was Sally's main love interest. Her background as a highly trained weapons and security expert, combined with her role as 'the woman' for the sake of the mission, meant that she was irresistably drawn to men in uniform.
Although appearing only as a guest star with a recurring role, Jan Hooks had an important and prominent role as Vicki Dubcek, the landlady's daughter and Harry's major love interest. She appeared in seasons 2 to 6. She even ended up marrying the Big Giant Head making her effectively Queen of the Universe. She also bore his child.
The Big Giant Head himself was also an important character and a commanding presence, even if he was rarely seen in the earlier episodes. When he did appear in person his role was played by William Shatner.
Another group of actors appearing throughout the series were the Pendleton Students. Four of them appeared in the opening episode and right through to the end. They were:
David DeLuise whose character's name was Bug Pollone. We were even introduced to his family in an episode that involved Dick having casual sex in the back of a car with his mother. He then proceeds to brag about it everywhere including to his class.
Ian Lithgow played the dim-witted but likeable Leon. One sometimes got the impression that he was smarter than people gave him credit for; that he acted dumb because people treated him that way. The role was played by the son of John Lithgow although the character of Dick generally treats him like an idiot. In fact he is often instigating the abuse.
Danielle Nicolet played Caryn, ardent feminist, activist and generally sensitive type who was usually the butt of Dick's male-chauvinist condescension and ridicule.
Chris Hogan's character's name was Aubrey Pitman but usually addressed as just 'Pitman'. He was the guy with glasses and long hair. Of the students, Bug and Pitman are particularly close friends. They seem to have known each other from before college.
Other actors had recurring roles in the series or had establised themselves as regulars by the end of the show's run.
Two women had an important bearing on Tommy's role as the sexually frustrated teenager. Shay Astar played August Leffler, his first girlfriend. She appeared in Seasons 1 to 4. In her final appearance Tommy had met someone new and she struggles to remind Tommy that she in fact was the one who ditched him.
The new girl in Tommy's life was Alissa Strudwick played by Larisa Oleynik and she appeared in Seasons 4 and 5. And a brief appearance in season 6 at which point they had broken up, mainly because of Sally's interference.
By the final season Joseph Gordon-Levitt himself was appearing only sporadically. That was probably due to other acting commitments which included the 1999 movie 10 Things I Hate About You, a modern rendition of Shakespeare's The Taming of the Shrew. Both Joseph Gordon-Levitt and Larisa Oleynik had staring roles though today the film is probably best remembered for being Heath Ledger's breakthrough.
Possibly arising from his obsessional devotion to Mary, Dick relations with the rest of his Pendleton colleagues are quite generally poor.
First of all there was Dr. Judith Draper played by Ileen Getz, professor at Pendelton, colleague of Mary but bane of Dick's life. She was initially introduced as a 'large unpleasant woman with sensible shoes' but later episodes reveal an altogether more complex individual. In one episode she is heard to declare: 'I love Bolivian men, they're like Honduran men without the attitude'. She is also seen to engage in flirtatious and/or amorous behaviour on different occasions with both Don and Tommy.
At the same time that Alissa Strudwick arrives on the scene it is also revealed that she is in fact the daughter of Dr. Vincent Strudwick, played by Ron West. This puts a strain on their relationship because Strudwick proves something of a rival to Dick with the two frequently coming to blows over professional and personal animosities.