I can't remember where I saw or read it (did E! have a THS about Night Court?), but I vaguely recall an interview with Markie Post where she said the producers were looking for a new public defender and her guest stint was like an audition. She obviously didn't get the job since Ellen Foley did. Markie said she was going to another job when she got the call asking if she would like to play Christine permanently for Season 3 and she accepted and the rest is history.
To stay on-topic, I went through Season 1 so fast I was hoping Season 2 was included. I really REALLY hope more sets come out this year (I'm not getting any younger). I just can't get enough of Night Court! My wife, who is not an avid TV or movie watcher, admitted that she really enjoyed it because it made her laugh and she liked the humanistic side of the stories. -
What actually happened at the beginning of season 2 was that Reinhold Weege created a new character, Christine Sullivan, and offered the part to Markie Post, but she was under contract to the show The Fall Guy. So Shelley Hack (of Charlie's Angels fame) was cast in the part of Christine. However, she was fired before the season actually began taping, reportedly due to her just not being very good.
The first few episodes of season 2 instead used guest characters in the public-defender role while they looked for a full-time female lead. One week Markie Post was able to get time off from The Fall Guy, so Weege wrote her into the show as Christine Sullivan, but as a guest character. Then she went back to The Fall Guy and Ellen Foley's Billie Young became the female lead. Then when Foley left Night Court and Post left The Fall Guy, Christine became a full-time character on NC.
My own ideal cast would be Christine as the female lead (though I like Foley), Mac as the clerk, and Selma as the female bailiff. Unfortunately Selma Diamond's death made that cast an impossibility (except in that one season 2 episode).