Each card’s powers reference specific situations, and if you’re not in those situations, you can’t play that card. If a card says it works on “any check,” you can play it on anyone’s check, but if it says “your check,” it only works on yours. You can’t play a Cure spell to reduce the amount of damage you’re suffering, because Cure isn’t about reducing damage. You can’t examine a deck that has no cards. Your weapon doesn’t help you acquire new weapons. Each card tells you what it’s for, and you can use it only for that.