Spaniards do not speak English
No seriously. In a lot of tourist places, they might speak English but they might not. They might not have English language materials. A lot of them will though. But still, you're going to a foreign country where English is not the native language. Know this, and don't expect them to speak English and don't get pissy, cranky and angry when they don't know English. Don't assume they are talking shit about you behind your back. You'll save everyone a lot of grief if you remember most people in the country have a native language other than English. Also, learning a few words of Spanish (or Catalan or Basque), or more than a few words, will endear people to you in many cases. Don't worry about a stupid accent speaking Spanish. American accents are sexy.
They speak Spanish at Starbucks in Spain. Yes, they do call Spanish 'español' in many cases, with español versus castilian being a preference issue but wider usage of español in tourist places. If they speak to you in English, it will often be limited and be about getting you through line faster.
While violent crime is very low, pick pocketing can and does happen.
Don't put out a giant sign advertising yourself as a potential victim. For example, last week I was on the train coming from the airport with some American university students. A college aged girl had a purse with a zipper. The zipper was partially open. The purse was located behind her back. She was not paying attention to her surroundings. She was just asking to be robbed. Her male buddy with her was wearing a backpack. It too had zippers. These zippers were not fully closed and you could see all his tasty tech toys inside. He also was not paying attention and could not see behind himself and was not positioned against a wall. Again, in tourist areas in bigger towns and cities, this is an invitation to get pick pocketed. Be on guard and take precautions.
Don't introduce yourself to Spaniards and then launch into your feelings on Catalan nationalism, Spanish Republicanism in terms of getting rid of the monarchy, Spanish politics, Franco, or Spanish racism.
It is fine to ask questions, but keep your opinions to yourself. It is generally rude to do this. Do you want them to respond back about how the US police constantly kill black people, how you all elected Donald Trump because you're that stupid, that Americans are so racists, that Americans can only play American football and suck at the real sport of football, and how Americans love their guns and the USA is unsafe to go to because you're a bunch of gun loving nut jobs? Yeah. I've seen stupid Americans fall face first into that one. Don't do unto others as you don't want done unto you. If the social environment is right, do ask questions because that will please them to answer in a lot of cases but keep your opinions to yourself.
Lots of Spanish foods are very local.
Know which region they are from before you talk about eating authentic Spanish food. Paella is from Valencia. In most touristy areas in the major cities, its 50/50 if you can get a decent authentic paella. And I don't know that I'd eat cocido anywhere but Madrid unless it was a Madrileño restaurant.
Callos. Offal that is super typical in Madrid and Asturias. Totally tasty and worthy getting locally made. Just don't ask what is in it.
Spanish meal times are not the same as the USA.
Deal with it. In general, meal times probably go like this: Breakfast: 9:00 AM - 11:00 AM. Second breakfast: 11:00 AM - 1:30 PM. Lunch: 1:00 PM to 4:30 PM. Merienda / Snack time : 4:00 PM to 8:30 PM. Dinner time: 8:00 PM to Midnight. Tapas, Aperitifs, Cañas of beer: 9:00 PM to 2:00 AM . In a lot of places, you might not find kitchens open for dinner until 8:30 PM and you won't be able to get warm food before that. Plan your meal times accordingly… and if you spot a place with a bunch of people happily eating dinner at 6:00 PM, that's a place full of tourists. It's not necessarily a sign that the place is a great place to eat because it is full of locals.
Also, do plan to have lunch as your biggest meal because Menú del Día is a really great deal. Don't expect free food with your drinks as the norm. They may bring you a small tapita of olives or bread or tortilla if you order a drink. In the case of Madrid, a lot of times, these small tapitas that are not olives are often leftovers from lunch or breakfast that they'd otherwise toss before the next day. It isn't enough for a meal. They can generally afford to do this because, aside from tossing them otherwise, they're making a fair bit off the drinks at €3 / coke or €6 / drink.
Menú del Día. Super typical. Super good deal.
Spaniards tip when eating out.
Not widely known, but Spaniards tip based on service. If your service was godawful crud, then you tip nothing. If it was the best service ever, 10%. If it was decent service, anywhere between not €0 and less than 10 percent.
Siesta is not a thing.
No seriously. Totally not a thing. Also, not a thing. Laura Hale's answer to How does the Spanish 'siesta' work? looks at this more, but copying from that: Siestas as they happen now are not three hours long. They are more likely to be 20 to 30 minutes. You don't want to get too much sleep as it completely screws up your sleep schedule. It's about recharging. Siestas are nominally built into the schedule for some places, with the time for this being generally between 1PM and 5PM. The issue is that for many people, this period of built in down time is also when they are more likely to have their lunch hour. They would have to sacrifice lunch for a nap. It just isn't hugely feasible. They may have other things they need to do, such as catch up on some work, do things at home, take care of children or parents, etc. They just don't have the time to take a siesta and there is no cultural imperative for them to try to do this. People most likely to do this are older people, unemployed people and children. The stereotype about lazy Spaniards having siestas just is not true. They work some of the longest hours of anyone in the European Union.
A worker takes a snooze in Seville. Photo: Karolina Lubryczynska/ Flickr