Suppose a cleric wants to question an NPC under the effects of a "Zone of Truth" spell. And suppose said cleric or a member of her party has the ability to strong-arm the NPC into agreeing. Can part of the "agreement" be that the NPC will not attempt to resist the spell? (that is, forego their will save)? This really comes down to 2 questions:
1) Can a creature willingly forego a saving throw on a spell where the description does not indicate that saves apply only to unwilling targets?
2) If a creature agrees to forego a saving throw, would it be possible to tell if they actually tried (or more importantly actually succeeded) a save?
And one more:
Is the answer different if the "request" is not made under duress? Suppose the NPC truly wants the cleric to believe him, and is willing to be questioned under "Zone of Truth" to convince him that he's telling the truth.