I've never personally had to take a polygraph examination before, but I have had a psychological evaluation before (which isn't all that bad at all). The psychological was just filling out a survey, you come back in another day, and the doctor asks you questions about the answers you provided. It was very simple and not mind-wrecking at all.
Can the same be said for a polygraph exam? Is the process similar in that you answer questions beforehand and then they ask questions about it, or do they use things you've put on your application? Really the main point is: will it always be something you've directly told them previously, or will they also ask questions about things they've found in your background investigation and other checks that you may not know about?