For one season I worked as a staff writer on a "cult" cable situation comedy ("cult" here is code for "briefly aired"), but before that I was a student of the sit com.
And I'm still a fan of the well-written sit com, although such seems to be at it's zenith no longer.
So I'm well-aware of one of the signs that a sit com has "jumped the shark" (gone into decline): stunt casting.
Stunt casting is the term that refers to casting a guest star in a part for which he or she may not be the best actor, but their notoriety or infamey is a desperate bid for more audience. The stunt cast also may be clutched at when the writer-producers have run out of good ideas.
The sit com employing stunt casting is usually exhausted and jonesing for an adrenalin shot to supply a cheap burst of energy, before inevitably falling back into torpor.
Stunt casting breaks the fourth wall: instead of moving the storyline along or adding to the suspension of disbelief, the stunt cast screams, "Look at me! I'm a distraction from crap!"
Just before it disappeared from the airwaves, my late '90s sit com brought in Gary Coleman for an episode.
'Nuff said.
Sarah Palin: Stunt Casting