Saeviomagy
Adventurer
In fact, come to think of it, I think the save every round mechanic for ZoT might be broken. It takes more than six seconds to answer a given question, so what if someone saves in the middle of a sentence?
Better, I think, to save per question.
My reading is that once a creature enters the zone, it starts making saves. Once it fails one, it stops making saves and from that point onwards cannot lie while within the zone. I'ts not an on/off/on/off save - it's a "while you stay here, keep making saves until you fail, then be subject to the spell" which is one reason why the spell gives so much wiggle room.
Anyway, my point of view on the spell is that it's the mystical and accurate equivalent of making someone swear in a court of law. You don't use it for the interrogation itself - you use it to confirm the results of that interrogation.