I write writing dialogue, probably a bit too much actually considering all the talky fics I seem to have written. I think the hardest and also most satisfying thing if you pull it off is trying to give the characters distinctive voices and ways of speaking without being too heavy-handed about it, and in fic that means making the characters "sound" like themselves. If I manage to do that, in my own mind or anybody else's then that's always very satisfying for me.