Hi hezzie!
I try to add the right amount of chili powder (and/or fresh chilis) to begin with, but if I have to add some heat afterwards (usually doesn't happen), I will just add some more chili powder (without frying) or, depending on the dish, more fresh red or green chilis - either whole or as a paste. I would put a cup or glass with selfmade chili flakes on the table, so everyone could add some heat if he/she feels the need to (: