I go by "be polite, be efficient, have a plan to kiss everyone you meet". With police, that means sound cooperative, but make it clear you don't really want to share information about anything unless you legally have to. If I'm asked why, my answer is that it's what my patent lawyer told me American custom is. If you do want to talk to the guy, remember to always talk to the person rather than the hat. If you're asked a question, answer it then ask something irrelevant/friendly right after. You want to interact with the quarter-italian guy who misses his grandmother's pesto and actually has a use for your pesto recipe, not with the law enforcement officer. I've actually gotten out of a parking ticket because the meter maid asked me what I did for a living, and I happened to have some prototypes in the car, and went into salesman mode until she told me to go away, her head swimming with way too much information about Android-based robotics.