I think it totally depends where you are from. I’m from the US, I just married a Spanish citizen, and had no problems, no interviews, no questions. This is only because I am from the US (they told my wife that if I was from a poorer country, they would interview).
I went to the consulate to get the paper saying I was not married. There was a fee to have the person in the consulate notarize the document.
You will need your birth certificate stamped with the apostille from your own country, and they recommended that my birth certificate had been printed in the last 90 days. Then we went to the local judge to sign all the paperwork (along with two witnesses). Later there is a waiting period, and the marriage petition is posted in the town/city. After the petition is accepted, you can get married. We got married in a civil ceremony (need 2 witnesses).
The whole process took a couple of months. Then comes the fun of filling out all the paperwork and the waiting in line to get your residence card (NIE).
I came to Spain in early January, got married February 22nd, and I will get my NIE card in about another month from today. Just to let you know it’s not a fast process.
Tyson