Ooo, tough one. I'd probably say be agreeable. Anyone you talk to wants to feel like they're right even if they aren't, they're the authority and you need to impress them. Unless your integrity is on the line just be agreeable and don't cause any waves.
So what's the situation for the question you're asking? If your interview is going smoothly like a natural conversation, then I would try to remain agreeable. Don't rock the boat.
[EDIT]
lol Spice, I was making my post before you posted yours. What a coincidence . . .
The question is:
During an interview is it best to be (1) completely agreeable with your interviewer even if you don't completely agree, (2) disagree with what they say and give your opinion if you don't completely agree, or (3) avoid a straight answer if you don't agree?
I have been to 6 interviews I think and was never faced with anything controversial. Besides AZ, that was just odd, but still it wouldn't help you to offend anyone. Be agreeable, kiss ace hard, sell it man!