Last time I was in a US theater the tickets where not numbered and you could sit anywhere. There was no point in pre purchasing a ticket because if you wanted a good seat you needed to show up early either way.

In Switzerland the seats have always been numbered and even if the cinema is empty people wouldn't dare move into another seat. People do show up right before the film starts and try to avoid the ads. Some also hang in the lobby until the film actually starts.

My experience in Geneva and Lausanne was certainly a bit more flexible regarding moving around the seats, unless this changed in the last 20 years.