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In general it is advisable to arrive early, as most theaters do not reserve seats. If you arrive late, an usher might show you the way. If not, say quietly "Entschuldigung" ("I am sorry" is also understood) to the people in the seats you are passing and try to reach a seat ASAP. Most people resent being stepped on, or to be shown your backside, as you pass. Face them, anything else is considered rude.