 With five months devoted to casting for the pilot , Gough and Miller cast ultimately hired eight actors to take on the role of series regulars for the first season . Since then , only two characters from the first season have remained regulars through to the eighth season , with seven new actors taking on lead roles from seasons two through eight . Four of those new actors began as recurring guests in their first seasonal appearance , but were given top billing the following season . As the series progresses , recurring guests appear at various times to help move the overall storyline of the show or just provide a side @-@ story arc for one of the main characters , such as Brainiac or Adam Knight . Other recurring guests appear as background characters , showing up for only a few scenes , which includes characters like Sheriff Nancy Adams or Dr. Virgil Swann . 
