Mrs. Stevenson is a character who eventually evokes quite a bit of pity from the audience. At first she seems to be a wealthy, perhaps spoiled, lady who lays around and gossips on the telephone. Later, we learn that she is really a lonely, bedridden shut-in whose husband is planning her murder.
This murder plot is revealed when Mrs. Stevenson attempts to use the telephone, and the wires somehow get crossed. She, along with the audience, is slowly pulled into the plot, slowly understanding that the mastermind is her husband, and slowly realizing that she is the intended victim.
Most people can empathize with a feeling of helplessness and vulnerability, and most people are afraid of being alone at night and suddenly finding that somebody else is there! As a result, the audience identifies with Mrs. Stevenson and feels both pity and fear for her.