The result should be "fall-off-the-bone", tender chicken.
Chicken is called "gà" in Vietnamese.
If you can't find galangal (it can be found in most Asian markets), use double the amount of ginger. Galangal is whiter in color than ginger but it's much more fragrant.
Serrano chile adds a nice aroma to the gravy. A small incision shouldn't bring spiciness to the dish, unless you bite in it of course!
I used water chestnuts for a crisp, crunchy contrast with the thick curry sauce. You can find fresh water chestnuts or the canned version in most Asian stores.
Thai basil tastes very different from sweet basil. It imparts a strong, earthy fragrance to the dish.
All the ingredients listed above can be found in any Asian stores.