The introduction of Mrs. Johnson was hilarious. We first met her in the diner after Bill Malloy was killed, and she ordered coffee from Maggie. Maggie tried to talk her into ordering a sandwhich and Mrs. Johnson went on and on about the mayonnaise being fresh; she always made her own fresh mayonnaise, Maggie tried to assure her that the mayonnaise was fresh. Well eventually Maggie brought out the sandwhich - Mrs. Johnson took one bit out of it, made a face and put it down.
Also the scene where Elizabeth interviews Mrs. Johnson for the housekeeper job was equally funny. It seemed more like Mrs. Johnson was interviewing Elizabeth than the other way around.
However, one of my all-time classic favorite humorous scenes was between Roger and Elizabeth, who went to inspect the room in the East wing where Vicki had been locked up by David (and seen the ghost of Bill Malloy). The siblings found seaweed on the floor where Vicki claimed to have seen a ghost. In uncharacteristic frankness, one turned to the other and point blank asked if they'd ever seen a ghost in their lives. The reaction was basically "I haven't if you haven't." They snapped right back into their usual state of denial and walked out of the room.