Hello there!
The answer to this question is costumes.
Why?
The audience is the people that are watching the performance, so an audience is a must. You perform for the audience, right?
Performance space, of course, is a must too. Without performance space, how will you perform for your audience?
Theaters must have at least one actor. Without even one actor, how can you perform for your audience? So the audience are just sitting there for nothing?
However, costumes aren't necessary in theater. You can just stand there and sing or dance and it won't affect the performance.
I hope this helps!
