This is a common conceit in video games, the companion is basically given you some discrete advice/direction but the animation engine doesn't have the tools to adequately show a conspiratorial whisper here and there.
Actually, come to think of it, this is a conceit that has been in stage plays for centuries as well. In stage plays it's due to the limited space of the stage and the necessity for the audience to hear the discrete conversation. In this case, it is because of limits of the engine.