Usually they tell you why the pulled you over, and if they say you look nervous just tell them you can't afford the ticket because your late on bills and this is just another thing to worry about. Best plan no matter what is to learn not to act nervous, be polite and try not to bull shit them. If you act evasive or like you are hiding something they will dig deeper.
I got stopped for speeding on night by the state trooper, he asked me why i was speeding and I told him i was being stupid and didn't realize he was there. he laughed went to his car ran my name came back wrote my ticket for less then i was actually speeding. Told me to be careful and slow down. Little did he know i had been drinking about an hour before and had a joint in the ashtray. It all depends on how you handle it. Cops are trained to pick up on our reactions so if you act calm and honest hey it's a quick routine stop and nothing more right?