Stbassy

I've just started this game, and I've read there are NPC quests that affect loyalty... I was wondering, do I have to use an NPC on specific missions? Or use them for X number of missions? If so on either question, will I get in game hints when where I should bring a specific character? (Preferably, I want to just use my favorite chars, but for completion sake, I'm guessing I'll have to swap someone out eventually)