I have Global Entry, so I don't know what the standalone Precheck interview is like, but mine was dead simple. Can you prove you're a US citizen (bring a passport if you have one), are you a citizen of anywhere else, have you ever been in trouble with the law? In return, it's saved me vast amounts of time (I'm a college student and fly several times a year to and from school) and stress. I ended up interviewing at an airport nowhere near where I lived, and they did ask about that; they were satisfied with "I can't get an appointment back home for six months, and I was passing through here anyway". I was approved pretty much as soon as I walked out of my interview, and my card arrived two to three weeks after my interview.