If you mean willpowers they activate under certain conditions. Most of them you need a certain amount of SP for between 100 and 200, some need you to be at 25% of max hp, others are always activated etc. You can read the conditions if you go to the status screen and pressing triangle (for PS3, no idea for Xbox) on the character of choice.
Also I forgot, when they are active the name turns green, you can check in battle as well if something is active with the same method as in your status screen. You can also check on the enemies willpower like that.