score:5
If you don't want to, or don't have enough time to, leave the airport at Bangkok, you can just stay air-side and never go through Immigration and Customs, but there are some issues to keep in mind, if you book separately:
Edit: your profile suggests you are in Sri Lanka. Are you leaving from Colombo originally? Thai Airways and Sri Lankan Air both offer direct flights from Bandaranaike International to Suvarnabhumi at very reasonable prices.