Lighting

http://en.wikipedia.org/wiki/Stage_lighting
To me, lighting is the most important technical facet of a production. Without it, we would be singing and acting in the dark. Lighting does a lot to contribute to a play, but I never knew that it could actually impact the audiences perception of what was going on. Say there was a scene that involved a fight; if its in soft lighting, it may seem out of place or weird. If there is a red light or a bit harsher lighting, the scene will appear more cohesive.