Yup, confidentiality is a big thing, however I can discuss a few things.
The day will be VERY long. Bring a lunch. The polygraph interviewer will even probably show you how accurate the test is by doing test questions to calibrate it , etc. Trust me, DO NOT LIE at all.
Here is the analogy that my polygraph interviewer used.
"Say your brother comes home from school before you do and sees a nice cake that your mother has baked. He and his friends decide to mesh into it and eat it all, leaving some icing and a few crumbs. You come home shortly after, lick the icing and have some on your mouth. Your mother asks you did you eat the cake and you say no."
Here, you would fail the question if hooked up to a polygraph, because the polygraph is so sensitive. They're not interested in your voluntary responses but your involuntary responses. There is no hiding from it, don't believe anything you see on the internet. THere are failsafe devices, etc. I challenge anyone who thinks they can beat it, to actually take it.
But about the above story, the moral really is not to leave any 'crumbs' on the plate. If he/she asks you a question, tell all and then some more. Elaborate to great details on EVERYTHING. It will only save your bum.
I found the test to be the most mentally exhausting thing I've ever done. I won't go any further because then I'd be getting into things that I agreed not to discuss with others, but if you were truthful so far and do not hide ANYTHING during the test, you'll be fine.
Trust me, even if you've smoked weed or stolen things in the pass, fess up and explain how you went wrong, and you admit it wasn't the best thing to do at the time, they understand people aren't perfect.
Trust me, if I could stamp something into your brain, it'd be "BE TRUTHFUL ABOUT EVERYTHING". You'll be fine if you do that. Trust me.