NPC quests

For NPC's that take time for their quests to pop up, do they need to be in your party? Can you do their initial quest, drop them, pick them up after a certain amount of time and their personal quest will trigger? Or do they need to be in your party for an allotted time for a secondary quest to trigger. For example, Dorn's quest after Radiant Heart.
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Your example? Not so much. The problem is that Dorn doesn't like to be dismissed. I've never seen him agree to rejoin the party after leaving it in SoA. Fortunately, you can eat up an in-game week very quickly by just traveling around in circles.