Bangkok Airport Arrivals
Arriving planes will usually park at the airport terminal, although some international flights and low cost airlines may park at the tarmac. If parked at the terminal, passengers enter a lower level of the concourses, separate from departing people. If your plane parks on the tarmac, a bus will bring you to the main terminal building a short way from immigration. Follow the signs to immigration or to the transfer desks if necessary.
If you are flying on Thai Airways and connecting to a domestic Thai Airways flight, you probably go through immigration in Bangkok and customs in your destination if your baggage is checked through. However, you go through immigration between the international and domestic concourses rather than the main immigration counters in baggage claim. Go to the transfer desk to confirm the process for you and find out where to board your flight. However, always check with the airline to find out where you go through immigration.
If Bangkok is your final destination, or if you're connecting to a budget airline, after passing through immigration, proceed through to claim your luggage. Once you have collected your bags, exit through customs to the arrivals hall. Note that there are no customs forms to fill out. If you have nothing to claim, you can go through the "green" lane. However, customs officer do perform "random" checks on passengers going through the green channel. The thing most likely to attract their attention is cardboard boxes. Even if you're carrying gifts or samples of no intended commercial value, you should go through the red lane and you may well have to pay a duty on the import of your items. If you need to bring items into Thailand, you're best advised to send them ahead by courier service, and not hand carry them with you. It's just too much hassle.