A list of every guest star on 3rd Rock from the Sun with pictures. All celebrity guest appearances. Guest performers most often play special guest roles, but sometimes they even play themselves. More often than not, guest stars appear during what is known as "sweeps", when a television show's Nielsen ratings are scrutinized to decide how much the network can charge for that show's ad space. Sweeps take place in November, February, May, and July every television season. Unlike a "cameo appearance" in which a star may only appear on screen for a few seconds, and may not even have lines, guest roles typically have substantial screen time with story arches that can spread throughout multiple episodes.