While there are some outliers, Southeast Asia is a fairly conservative place and keeping in mind local sensitivities and cultural norms will help you avoid offending locals.
While there are some country-specific no no’s—like speaking in disparaging terms of Thai royalty in Thailand, say—there are some general etiquette ideas to keep in mind across the region.
Much polite behaviour in Southeast Asia is based around the concept of “face”, especially the acts of losing or saving face. While entire books have been written on this topic, at risk of oversimplifying you can boil it down to dignity and maintaining social hierarchies. Face can sometimes be a difficult concept for Western travellers to get their head around, but travelling in Southeast Asia, you will encounter people “saving face” regularly.
For instance, people giving you directions when they have no idea what they are talking about is all about them not losing face. People not telling you that you are doing something wrong or impolite is so that you don’t lose face—they don’t want to embarrass you. If you get visibly angry, you lose face, and are far less likely to get what you want than if you smile, even if seething, and speak your case politely.
Face is a social dance built around asking the right questions and giving the right answers.
The feet are widely considered to be dirty and impure, so avoid gesturing at something with them or pointing them at people. In temples, never sit with your feet facing the altar. Never place your feet on the back of another’s chair, such as on a bus or train. Related to this, you’ll often find people take their shoes off before entering homes or temples. If you see a pile of shoes, add yours to them! To not remove them when asked to is particularly rude. Wearing socks is fine.
Accommodation should offer some form of safe keeping for your shoes—don’t leave your expensive trekking boots out on the street too long and certainly not overnight.
At the opposite end of the body, don’t mess with people’s heads. While you could get away with affectionately tousling the head of a child, never, ever try to do that to an adult.
Traditionally viewed as unclean, you should never eat with your left hand, and try to avoid passing things to people with it.
While holding hands is generally considered to be acceptable, overt public displays of affection such as excessive kissing is very much frowned upon.
When at a place of worship, dress appropriately. Wearing a bikini or going shirtless to a temple is not okay. Men and women should cover their shoulders and knees. If you’re wearing some sort of spaghetti-strap top, at least wrap your shoulders in a scarf. Remove hats.
At restaurants, never leave chopsticks vertically stuck in a bowl of rice; lay them across the top of your bowl horizontally. Bringing alcohol into a Muslim restaurant is not okay unless you have explicit permission from staff. Likewise Muslim-run guesthouses may prefer that you do not drink in your room or in the common room. Generally, signage should help guests figure this one out.
Planning well is an integral part of getting the most out of your trip. Be it picking the right backpack, the right vaccinations or the right country, the simple decisions are often the most important.