How, exactly, does one do quest turn-ins to KOS NPCs?

[Afterburner]

I know there are methods which will let me turn in quest items to NPCs that would really otherwise like to kill me, but I've never had to do so on any character I've played. So I don't know the actual nuts-and-bolts of the process.

How can (for example) my Troll Warrior turn in DF belts to whats-his-face in West Freeport? How can my Barb Shaman turn in DF belts to the Troll Shaman guildmaster?

Inquiring minds wanna know.