Open seating, whatever the event, works on the premise of “first come, first served.” But as etiquette is too gracious to be taken literally, it overlooks the distinction between actual and imminent presence.
Miss Manners has no objection to anyone placing a coat on the adjacent seat while a spouse goes in search of popcorn; she (not to mention other ticket buyers) is less tolerant of large swaths of territory being staked out for acquaintances who are still at home, looking for the car keys.
Dear Miss Manners: An acquaintance's husband phoned and asked to borrow a large, expensive tool. I had mentioned previously to his wife that they could borrow it, so this was not completely a surprise. But it was a little surprising that it was he who made the call, when we have met only briefly, and that they wanted to borrow it immediately.
However, I said yes, and then hurriedly made a few rooms presentable, because we had been enjoying a weekend lie-in. Some spaces were definitely not presentable, and my acquaintance is extremely image-conscious.
They arrived, and the husband loaded it into their vehicle while she looked around the property, checking out a great deal of the house. It was her first-ever visit, so I understand her having a little curiosity. When she started to go into spaces I had not tidied, I asked her, "Please don't go in there."
She did anyway, and had a good look around. I did not apologize for the state of things, nor offer excuses, but I was embarrassed and also extremely annoyed. I don't know what more I could have done to deter her without being rude.
Step 1. While your acquaintance is investigating one room, physically place yourself between her and the next location you consider off-limits. This may be the next room or the bottom of the stairs. Since you have the home-court advantage, Miss Manners leaves the specifics to you.
Step 2. Offer tea, coffee or anything else that will move your nosy acquaintance to a more neutral location — while still allowing you to block her advance. If she refuses refreshments, ask her to join you in the front room (on the excuse of pleasant conversation).
If all else fails, begin whatever pleasant conversation you can muster without moving. No accusation of rudeness can be leveled against a hostess for where she stands while performing her hostly duties, unless it be in front of the bathroom or the exit.