<P> The Solomons encounter other seemingly alien beings, most notably, Jell - O, or when in one episode Dick and a friend of Mary's remain confused throughout--they know they are "different"--but Dick thinks "alien" and the friend thinks "gay". Their first encounters with snow and dreaming cause general hysteria . </P> <P> 3rd Rock maintained a constant ensemble cast; the four main characters are Dick, Sally, Tommy, and Harry . Several other main characters who left or joined the show through its original run supplemented these four, and numerous guest stars and one - time characters supplemented all of them . The three male aliens' names are a play on the phrase "Tom, Dick and Harry" which is a placeholder for multiple unspecified people . (When Don eventually notices this, they look uncomfortable and Tommy says, "Well, it's not like it's a deliberate attempt on our part to seem average," which is of course exactly what it is .) </P> <Ul> <Li> John Lithgow as Dick Solomon: Though the High Commander and head of the expedition to Earth, he is often the most childlike member of the group, being, ironically, the youngest of the crew, despite being the oldest family member (at least appears to be the oldest due to his body). Much of the behavioral or societal - based troubles faced by the crew in their mission while on Earth frequently arise from some juvenile act perpetrated by Dick, troubles which in turn are forced to be overcome by the entire troupe with a great deal of reluctance . </Li> <Li> Kristen Johnston as Sally Solomon: With a rank of lieutenant, she is the security officer and second - in - command . She has been called Dick's sister, but was sometimes introduced as Tommy's sister earlier in the series, and, on one occasion, claimed to be his mother, although never Dick's daughter and certainly not his spouse; failure to clarify the exact relationship between Tommy, Harry, and Sally led to humorous confusion whenever either Harry or Sally attempted to act as Tommy's guardian . Sally was chosen to be the woman because she apparently lost some sort of contest and was not too thrilled about it; while the alien species is described as asexual, Sally seems to have a harder time trying to figure out womanhood than the others do manhood . She files a request to be made male early in the mission, though later decides she liked being a woman . A later episode had Dick and her switching bodies on orders from the Big Giant Head, which caused much confusion both on their parts, as they had gotten used to their respective genders while on Earth, and the other characters, as they could not get used to Sally acting like Dick and vice versa . </Li> <Li> French Stewart as Harry Solomon: Originally, he was not part of the mission, but just happened to go for the ride because an extra seat was available . Later, it becomes known that a chip is in his head, and he becomes the Communicator or Transmitter . Occasionally, he gets a message from the Solomons' leader, the Big Giant Head, and shakes violently in the middle of a sentence and squats down, with his arms at 90 ° angles, declaring "Incoming message from the Big Giant Head!", before going through the motions of delivering the message, which always ends with a violent sneeze . He poses as Dick and Sally's brother, and Tommy's uncle . Harry is often known to be the simplest family member, is occasionally impulsive and misunderstands people . He is also prone to misfortune . Though he seems to fit in on Earth with the earthlings better than anyone else, this is implied because he is seen as dimwitted . Whether he is really dimwitted or just more sensitive to his surroundings is unclear, however . </Li> <Li> Joseph Gordon - Levitt as Tommy Solomon: Information officer and third - in - command behind Sally, Tommy plays the role of Dick's adolescent son, yet he is the oldest and smartest of all the aliens . Throughout the series, Tommy continually reminds the others of his superior intelligence and greater age . Gordon - Levitt left the series (after the fifth season concluded) as a primary character, only appearing as a recurring character in just over half the episodes of season six . In the show, this is explained by Tommy's graduating from high school and moving out to college . </Li> <Li> Jane Curtin as Dr. Mary Albright: Dick's colleague and on - and - off girlfriend, Mary is aware that Dick is an insensitive idiot, but appreciates his joie de vivre and quirkiness . Reference is often made to the insecurity caused by her bad parenting, and the fact that before Dick arrived, she was known for sleeping around, and had even been nicknamed "Dr. Slutbunny". </Li> <Li> Simbi Khali as Nina Campbell (seasons 3 - 6, recurring previously): Dick's and Mary's administrative assistant, she finds Dick beyond the pale . </Li> <Li> Elmarie Wendel as Mrs. Mamie Dubcek (seasons 3 - 6, recurring previously): Mrs. Dubcek is a "blast from the past", namely the' 60s . She boasts of having danced naked with Jack Kerouac in a cage . In addition to being the landlady, she is a friend to the Solomons . </Li> <Li> Wayne Knight as Officer Don Leslie Orville (seasons 3 - 6, recurring previously): As Sally's boyfriend, he is an underachieving, spineless police officer, who has a talent for romanticizing his rather mundane job . Sally is completely enraptured by his stories, and is reduced to simpering and giggling around him . </Li> </Ul> <Li> John Lithgow as Dick Solomon: Though the High Commander and head of the expedition to Earth, he is often the most childlike member of the group, being, ironically, the youngest of the crew, despite being the oldest family member (at least appears to be the oldest due to his body). Much of the behavioral or societal - based troubles faced by the crew in their mission while on Earth frequently arise from some juvenile act perpetrated by Dick, troubles which in turn are forced to be overcome by the entire troupe with a great deal of reluctance . </Li>

Who starred in 3rd rock from the sun