We’d like to begin with a short note about fast food and spicy food. Contrary to popular belief, thai cuisine is not meant to be hot. There are a great variety of herbs and spices used in thai cooking. These may add flavor, aid digestion, have medicinal properties, or just bring color to the food (usually it's a combination of a few qualities).
The types and flavors of herbs and spices vary as you travel from one place to another within Thailand. Just as anywhere else, there are people that cook and eat really hot food; however, almost all Thai restaurants will ask for your preference while taking your order. If they forget to ask you, please do let your waiter know.