If you are stopped: SIX SIMPLE RULES

3. When asked to exit the vehicle, make sure the vehicle is in park and exit the vehicle carefully. Don’t lean against the vehicle. 

Officers are trained to watch for several signs of intoxication as you exit the vehicle. Exit the vehicle carefully. If you refuse to exit the vehicle, you can be charged with a felony "resisting and obstructing." 
Here is a slide straight out of the police officer's field sobriety testing course materials describing the so-called exit sequence: