Whenever I complete quests or subquests, I immediately go back to town to sell items and see if one of my companions has something new to say.
However, it seems awkward that still, quite many dialogues from all companions seem to be missing. Example Screenshot:
What are the possible preconditions to unlock character dialogue? Do I have to have visited certain places (maybe dungeons?), or have done certain things, found certain lore books? Do I have to take follower's with me to certain events in order to unlock certain dialogue sequences specifically for them? What is it?
Also, do I have to meet these preconditions again when I'm starting a new game on a higher difficulty, or do dialogues stay unlocked?