If the play is dark and serious we expect the lights to help us feel dark and serious. When a character begins to sing, the music helps us get into the mood of the song. Before the show starts the crew must make certain that all of the equipment is working, the props are in the right places, and that the stage is set exactly as it should be. Observation of detail is especially critical for crew members. If a piece of scenery is out of place by a fraction of an inch it can cause a domino effect of missed positioning for the actor and in some cases can be dangerous.
Many plays have dance or tumbling elements which require a very specific spacing of elements.
The crew members must be accurate in their tasks at all times. Probably the most common things that people think of in maintaining the illusion are costumes and makeup. These help us understand what the characters are about, and to form expectations about them. Clothes, hair and appearance are our everyday way of judging people, though we claim not to do it. The skills involved require practice and as I observed the classes and performances I noticed the level of helpfulness among the students.