Over the summer I worked at a fair with a hypnotist and some other acts. One night during the hypnotist’s show, the drummer from the band that was next onstage started bringing his drum kit onstage and setting it up! This is a huge breach of etiquette when you are sharing a stage with another act.

The thing is when you perform at fairs there are really two types of acts, acts that are professionals and acts that aren’t. The community acts are typically acts that are newer and just haven’t ever shared a stage with another act and don’t know the rules.
While it’s annoying a hell when someone tries to set up during your show, it’s also a teachable moment as they obviously don’t know that they shouldn’t do it. When it happens during my show, I simply mute my mic and firmly say, “This is an active stage and you need to get off of it right now.“
No discussion, just that said firmly.
Then after my show, if they don’t apologize to me, which they will about 90% of the time, then I explain to them during someone else’s show 100% of the stage is theirs…unless other arrangements have been made. I usually ask/mention how they would feel if I went onstage and started setting up while they were performing.
Educate them!
-Louie