The textbook puts these two designers in the same chapter.(Chapter 11).Let's take a look at lighting designers first.

On page 243 there is a list of all the objectives of a lighting design. Some are very obvious and very simple: to light the stage! So we can see the actors! Others are often subtle and the audience may not be aware of how they can convey the mood, or even the focus of what they should be looking at. They clearly have to know what the set and costumes are going to look like so everything works together. On page 248 the photo essay shows the some of the lighting effects for different shows, and on page 250 the two photos show the same set can look with dramatically different lighting.

Some plays ( musicals, for example ) will have over a 100 light cues and changes. Lights can go straight to BLACKOUT or fade softly, depending on what the director wants. Blackouts happen often in comedies; fade outs more in drama where a gradual dimming is needed.

Let's again let the amazing designers of "Wicked" explain: