Virumor,
Any session that involves in-character conversation requires nothing but the ability of the players to adequately convey their character. Whether they are investigating, just talking amongst themselves or talking to NPCs, whole sessions can pass that way.
Sure, they will be part of a larger campaign (At least mostly), but the heart of role playing is freeform acting. The rules are there to provide a framework for the game, not to BE the game.
ANY suitable framework will do.
