SO2 PA 2
Private Actions can affect your romance and friendship points. Especially if there's a choice involved. This doesn't just affect the endings, but also determines how the AI will act in battle.
Say, for example, that Noel has a high affection rate for Opera and has an average rating for the other two particpants in the battle. Say that he didn't know any group healing spells or didn't have enough mp to cast a group healing spell. Whoever he has the highest affection rate for, in this case Opera, he would heal first.
I don't want to explain it all, so just go to Gamefaqs and there should be a faq there.