It's easy to think of it as a quick reaction to a physical situation. While courage is part of one's personality; the reaction is thought out and represents one's morals. For example, joining the army is courageous; attacking an enemy is courageous. Joining the military shows a piece of your personality. Attacking the enemy is only part of the job; it's not thought out or planned, it's a quick reaction. In opposition, perhaps the context calls for a word, courage, but courage perhaps sounds better. Using the other word is not necessarily wrong. Both terms fall under the same umbrella, but to be completely correct you should really think about the criteria involved in both