Many sitcoms are recorded in front of a live audience who provide the laughter track. Now this is simple enough in most scenes. But some scenes require added special effects. For example the dinner scene in Red Dwarf season 6 episode "Legion". Part of the comedy is the food hovering in the air, and part of the drama is the reveal of Legion's face as an amalgamation of the cast's faces. You can't do that live, and it has to be done in post production.
So how is that sort of scene filmed in front of a live audience?