I don't often go to the theatre, and usually if I do, I/we go for good seats, so the most I've done before is shift more to the centre if those seats aer unoccupied.
So I went to the West end theatre last week and it was officially a sold out show. I had a seat literally in the last row on the very very steep upper balcony, and this was between other occupied narrow seat. This was still not a cheap ticket at 25 pounds for that partial view! Behind the row there was standing room for 10 pounds, so some people were standing obviously.
I've noticed that there was a spare seat on the end of the row in row three - very restricted view but much lower down on same balcony and not squeezed between people - I moved there when they've closed the door. I could only see 1/3 of the stage if that so it was hardly anything premium. In the interval there was a man coming out of the middle of that row and I've heard him talking to his partner about seats, I wasn't really listening. As he was loud I looked at him and realised he was staring at me and talking (I only eventually clocked that he was talking, not just looking at, me or about me) - something about people being bullish to sit on the end of a row and there was more of a rant but I didn't listen to the first part. I thought he was being jocular about something so I smiled at him especially as he was older, to be polite, but he didn't smile back and then I realised he probably chastised me!
I mean, it was an empty seat. Is it seen as unacceptable to sit anywhere slightly better than your seat? I think people assumed I had a 10 pound standing seat as some did give me looks, so he was probably outraged that I took a 25-pound one but that was what I paid - I just wasn't going to prove anything to others. Plus I am a bit surprised that people who were standing behind didn't take couple of other spare seats even if they did pay for standing - I wouldn't mind that at all as a higher payer!