Summarize:

Place 1 US tbsp (15 ml) of sour cream or yogurt into the pot of curry. Mix it in with a wooden spoon. Taste the curry to see if the spiciness has been reduced.  If the curry is still too spicy, continue adding a spoonful of either ingredient and tasting it to see if it improves the flavor. Be careful not to add too much sour cream or yogurt, as this may make the consistency of the curry too thick. Many curries have a coconut base, which means that adding coconut milk can work perfectly to dial down the heat. Add 1 US tbsp (15 ml) of coconut milk to the curry sauce, and taste it to see if it has helped. Simply add another spoonful or two if the first wasn't enough, and keep tasting the curry to see if the flavor has improved. Milk can also help to tone down the spiciness of curry. Pour 1 US tbsp (15 ml) of milk into the pot that has the curry sauce in it, and mix it in to combine it with the other ingredients.  Taste the curry after you add the milk. If it hasn't reduced the spiciness enough, add a bit more and try it again. Don't add too much milk at one time, as it could change the consistency of the curry and make it watery. Whole fat milk is the best to add to curries. Use either an oil that tastes neutral, such as vegetable oil, or one that will complement the flavors in the dish. Only add 1 teaspoon (4.9 mL) of oil at a time, and mix it in thoroughly. Always try the curry before adding more oil to see how it has changed the taste.
Add sour cream or yogurt if you don't mind thickening the curry. Use coconut milk if it matches the flavor. Add milk if you don't mind thinning the curry. Use extra oil if the recipe already includes oil.