What is a person being shown something called?
Let's say I am demonstrating to a person "how to flip a chair" by flipping a chair for him. What is the person who is being demonstrated to called?
That person would be called observer.
What is observer? Well, a person who watches or notices something.
Audience could also work in some contexts, especially if you are doing something to delight or entertain others.
E.g. David's card tricks were a hit at the party, because his audience was willing to go along with his silly antics.
If it's a demonstration, the viewer could be the -- viewer.
How about a viewer or a spectator?