It's best to ask permission if you want to photograph someone, especially if you are working in close. Engage them before you pull out your camera. Learn at least how to say "hello" and "May I make a photograph" in the local language—just showing that you've made a little effort helps. Explain to them what you want to do and what it is about them that made you want to make a picture. If approached in an open and friendly manner, most people will be agreeable—many are flattered that someone has shown an interest in them and what they do. In places where there's a lot of tourism, you may run into people who are tired of being photographed—many tourists are not courteous enough to ask permission, and local people can come to feel abused and exploited. The only way to overcome this is to spend time with the people or to go to parts of the place less frequented by tourists.
Rosalynn Tay is a travel and fashion (as well as editorial) photographer based in Singapore, and is a peripatetic traveler whose fondness of travel led her to photograph in countless countries. She travels to Sri Lanka, Japan, Mongolia, Bangladesh, China, Malaysia, Siberia, Morocco and even ventured to North Korea. She is a graduate of Spéos, the internationally recognized photography school in Paris. She's also a committed Leica user, and has exhibited her work (Ethiopia -solo- and LeicaXhibition -group). 
I have found Alessandro Bergamini's China gallery to hit all the right notes in that regard. Most of his images were made in Guizhou; a province located in the southwestern part of the country, and well known for its traditional rural villages, inhabited by minority groups like the Miao and Dong. Other images in the gallery were made in Guangxi , another autonomous region bordering Vietnam, and home to the famous cormorant fishermen of Guilin. I haven't been but I read that even though the water is too polluted now for fishing to be sustainable, these fishermen are catnip for many photographers, and earn their living in that fashion.
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