Companions and henchmen can talk to NPCs in order to trade, but the only actual dialogue is the NPC saying they don't want to speak to underlings.
They can pick up items, etc, and trigger combat, but there are some things that need to be triggered by a main character.
I haven't played co-p at all, so I'm not sure about listening in on conversations (or the conversation log), or the 'anything else'.
