I would imagine the sobriety, and to gauge your demeanor. If they ask you a question, and you respond, they can assess whether you are likely to assault them or fight them in some way. guilty people tend to scream and yell and become defensive. Innocent people are just non-chalant, like, "Hey, what's up?" I once got pulled over for speeding and told him straight out, "I was speeding. Sorry." Then he told me he wanted to check out the inside of my sports car. He was nice. I was nice. I got a ticket, but so what? I was in the wrong, and he did his job. Officers have a tough job, not knowing whether a routine pull over could turn into a shootout.