Government Polygraph Testing
For particularly sensitive information, candidates may have to undergo government polygraph testing. It's not quite as dramatic as "Moment of Truth," that Fox reality TV show in which contestants have to answer polygraph questions about whether or not they have ever wanted to cheat on their spouses in front of millions of people, but for most, it is still a fairly stressful experience. The candidate must be connected to a polygraph machine, which measures fluctuations in heart rate & other biological "tells" that could indicate lying.
The first "layer" of polygraph is simply a matter of verifying the answers to the extensive list of questions you have provided for your background check. It covers all the same material: relationships with foreign nationals, excessive debts, drug use, a desire to hurt the U.S., and things of that nature.
The Lifestyle Polygraph is not required for all positions, but its purpose is to dig out any skeletons in your closet that some spy could potentially use to blackmail you should you ever be placed in a position of public trust with access to very sensitive information. Think of the made-for-TV examples of activities for which people are blackmailed - these usually consist of people trying to lead a double life and hide their activities from the world. Investigators will ask about drug use, alcohol abuse, gambling habits, homosexuality, soliciting prostitutes, having affairs, you name it. These are not easy things to talk about. Many people become very offended and angry that someone even has the gall to suggest that they would do such things.
As with everything else related to the background investigation, simply being as open and honest as possible will make this whole process go smoother. Understand that the process is designed to dig up skeletons, and that the investigators' job is to prod. Then take a deep breath and just plug through it.