The Spanish Police are professional and polite but firm. They want you to know they are in charge and not you. F around with them and they will F you up.
I have been to Spain many times and never had a problem. I was always polite and they were always very professional but firm. If you are a foreigner and speak Spanish they are very pleasant. Last time we went to Spain we spoke no English but rarely. We stayed in hotels where middle class Spaniards stay. I speak Spanish and my wife's first language is Spanish which we speak at home a lot. Spain is a great vacation and the food is excellent. Spaniards are very serious about their food!
I forgot to add. I would suggest one visit the Picos de Europa in the North of Spain. Absolutely beautiful is this area and not a lot of tourists as compared to Madrid and Barcelona and the Mediterranean coast of the South of Spain. Also, they have great beaches on the Atlantic. The water is really cold as opposed to the Mediterranean, which is warm.
My wife and I selected a modest hotel to stay in and almost everything was conducted in Spanish. The clerks spoke some English but our Spanish was better than their English. We stayed there for a week. Breakfast and Dinner was included. We made day trips to surrounding areas. It was a grand vacation.
The problem many tourists have is trying to see everything via organized tours of which you become just passengers on a bus and it all becomes a blur and where in the hell is the next bathroom.
Find an area you want to see and a moderate hotel. Explore the areas within a two hour drive in your rental car you picked up at the Madrid or Bilbao airport. You will be pleased.