Blending in respectfully rests on a handful of behaviours that work in every country, topped up with destination-specific homework. The universals: greet before asking, since transactions without greetings feel cold or rude in most of the world; dress a touch more conservatively than home until you have calibrated to locals; keep your voice moderate in public; show visible respect to elders, religious sites, and national symbols; be patient with queues and bureaucracy; and thank people in their own language.
Then learn your specific country's big levers, because they differ. Key questions to answer before arrival: Are shoes removed indoors? How formal are greetings, and is cross-gender contact avoided? What are the dress expectations, especially at religious sites? Is the culture direct or face-saving, punctual or relaxed about time? What topics are politically or historically radioactive? Is tipping expected, optional, or awkward? A reliable culture guide answers all of this in an evening.
After that, observation does the rest: watch what locals do in each new situation and copy it, ask local friends to correct you and mean it, and when you err, apologise sincerely and adjust. Effort is legible everywhere, and locals worldwide are generous with foreigners who visibly try.