There is a lot of "theatre" around that isn't "theatre," as it makes no use of the audience's imagination. There's nothing to do, except sit there until you leave. You look at a set of a living room, and it's all there, down to the last electrical outlet. So we are in the habit of suspending less and less disbelief. Occasionally, an actor in a desperately naturalistic play will switch on a lamp beside a bed, and the lamp (controlled by the stage manager) will come on a second early or late. Suddenly, there's this feeling, this life, real honest laughter in the audience, even self-awareness, somehow. So...it isn't theatre if the lamp comes on at the exact moment in which the actor pretends to turn it on.
-Will Eno, twenty questions with American Theatre