Hey,
this question has probably been answered elsewhere but I couldn't find it via search so again:
NPC-NPC Interaction in Skyrim was one of the things that contributed most to the "alive" feeling cities had. But since there are already 50.000+ lines of dialouge that need to be voiced, will there be addidional lines written to have some sort of dialouge between NPCs only?

There was a big project last
There was a big project last summer that involved the writing of somewhere between 7,000-10,000 new lines of dialog for just this purpose. Almost every NPC in the game has at least one conversation with another, so there will be plenty to listen to once Skywind releases.