We were told when abroad that people would always be able to tell we were Americans, but that - depending on the country - we would do well not to make it too obvious.
What was strange to us was to be in any way friendly to strangers. Eye contact, smiles, even things we might consider courteous. Definitely do not "chat" with strangers unless they initiate. Wear shoes, not sneakers. Don't wear shorts.
But every country had "rules" we didn't know about, and it made us "rude". In Ethiopia, it was inappropriate to ever eat with your left hand or to reach across a table. Most places, it was rude to show the bottom of your shoe. On the other hand, everyone appreciated even flailing attempts to speak their language.
Well, except the French, but - you know.