Some of these clichés exist because, to be emotionally involving for the viewer, some fundamental element of the characters' life needs to be in danger, but cannot actually be changed permanently because of the risk of breaking the carefully-formulated, situation-generating premise. So something big and important happens at the beginning of the story, but is resolved before the credits. In conversations about sitcom writing, I refer to this by saying that the events in question "cancel out of the plot." (In novels or movies, characters, situations and events which cancel out usually should have been removed. In sitcoms, most episodes cancel out because otherwise the premise would change.)

Because of this, fundamental changes in the lives of characters happen, for good or ill, at most, very rarely. Also, if some important event were to happen in a particular episode that would change the premise, it might break the continuity should that episode be shown out of order. Because of this, important episodes usually occur at the beginning of the series (setting up the premise), at the beginning of a season (because of actor death or firing, or major retooling during hiatus), at the end of a season (Can you say cliffhanger? How about two-parter?) or at the end of the series (Goodbye, Farewell, and Amen).

Because of the delicate web of tension (or various sorts) strung between the characters in a sitcom, the balance of power between characters does not change much over the course of a series. This is something to think about should you ever realize how much roleplaying games resemble situation comedies.