Keep two things in mind when it comes to what to expect with public transport in Southeast Asia: the standard of driving and how you should carry yourself.
The standard of driving can be appalling. Minibuses in particular are notorious for terrible driving at high speeds. The internet is awash in dash-cam videos of plainly insane minibus driving. Often overloaded, accidents and resulting fatalities are not uncommon.
Long-distance, overnight bus drivers likewise often demonstrate extremely poor driving abilities. Barrelling around a blind corner while overtaking other traffic is commonplace. Ideally, you want a seat on the non-driver side of the bus about half way down the length of the bus—we figure this is the least likely part of the bus to see a direct impact. We don’t catch night buses much. Buses can increasingly be booked online and in some cases specific seats can be requested.
If you need to stow your luggage in the luggage compartment of a bus, never leave valuables in the luggage. Theft from bags is a major problem. Keep your valuables with you in the cabin of the bus.
Southeast Asia's train network has been improving over the years, but many trains still have both doors and windows which can be opened while the train is in motion. Making use of your common sense and not hanging out of the train while it is moving is generally a good idea.
Airport and flight security may appear to be haphazard and randomly applied. Don’t be surprised when half the passengers stand up as soon as the flight touches down.
Transport often leaves ... when it leaves. It may be when it is full, at 09:00 as per the schedule, when the bus driver is ready, or just when it feels right. This can be infuriating at times, but there is little you can do about it—screaming at the driver won’t help. Be wary of daisy-chaining together elaborate planes, trains and automobile trips which will fall over with one missed connection. Be especially wary of relying on ground transportation for a tight connection to a long haul international flight (in other words, don't).
As far as behaving yourself is concerned, keep in mind local cultural norms and sensitivities. Easily the most commonly complained about behaviour by foreigners is them putting their feet on the seat—especially onto the back of the seat in front of them. In Buddhist-majority countries, pointing the soles of your feet at someone else is considered to be the height of rudeness. You can't get much ruder than sitting with your feet planted into the rear of a person's headrest.
Likewise, public affection is not encouraged. Holding hands, a quick peck here or there is no big deal, but totally making out halfway down the bus? Save it for the room. It will be extremely unlikely someone would come and say something, but it isn’t appreciated. Not surprisingly, drunk and disorderly behaviour on public transport (or anywhere for that matter) is also frowned upon.
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.