I usually just call for initiative when either the players or npc's/monsters do or say something that triggers combat. In PotA, I had the same situation with the players inquiring about the stolen books. when the players wouldn't leave it alone and kept pressing him I said "you see his mocking smile fade as he begins uttering a incantation, role for initiative"
then the combat plays out from there as usual.
then the combat plays out from there as usual.