I saw a very powerful production of "The Diviners" years ago.  It has stuck with me for this long because of one thing:  sound design.

When the preacher takes the little boy to the river to overcome his fear of water, the boy "goes under" the water.  This was done on a highly-raked stage so the "shoreline" was at the top and the "river" was at the bottom. 

The presence of water was merely suggested by clever lighting (the rippling effect of lights off a swimming pool) and sound (the trickling and splashing of rushing water).

When the boy "goes under" the gentle trickling sound was suddenly and shockingly replaced as the audience was blasted by a mixture of sounds -- the muffled wailing, sloshing of water, muted voices, a heartbeat, etc -- all blended into a cacaphony that one would imagine hearing under water.  The other actors were literally "drowned out" by the noise.  The sound would cut out as the boy thrust his head "above water" and gasped for breath.  Then the sound would resume as he went under again.

It was horrifying, fascinating, absorbing and completely believable.  One audience member lept from his seat in an effort to save the drowning boy.

I'll never forget that play as long as I live!