This thread is used to chat or interact with NPCs and other player characters.  Use this thread for any sort of back and forth between multiple people that isn't directly moving the story forward.

 For example, if a new NPC joins the party a basic introduction will be given in the main thread.  If you would like to talk to them more, this would be the place to do it.

The chatting and interactions here are assumed to take place in the background, and for the most part won't have any definite timing linked to the main thread.  So you could use this thread to be carrying on a conversation while the main game thread is in the middle of a battle for instance.