My experience with police has been simple.
Answer the questions. Be honest, Be direct and keep it short and sweet.
"yes sir", "no sir", "i believe I was going 55mph sir", etc...
In the example of the guy saying "i have nothing illegal in my car". Well - that's fine, but that's not what you were asked. You were asked if there's weapons in the vehicle. That's a simple yes or no question. His answer would have raised suspicion with me as well - because his answer didn't really answer the question.