I've talked to someone who administers polygraphs, and they said the machine is mostly theatre, but that there is normally someone watching you through a hidden camera or double sided mirror to see how you act when you think someone isn't looking. For example, when I had to take a polygraph for work, the polygrapher left the room a few times because they "needed to get a second opinion about abnormal results". That's when the hidden person would supposedly be watching to see if I acted differently when I thought I was alone.