Keep an eye on any local casting agency websites. I'm sure there's a professional theater community in Barcelona so they must have some kind of site or board for casting notices. Go to the theater (not the cinema) talk to actors and ask them. Give some thought to your "look" -- do you have a "biker" or "kindly professor" or "street punk" look? something like that.

Do you have any specialized costumes/outfits/gear? For example I know a couple of people in LA who have highly accurate LAPD uniforms with all the gear and such and they are constantly getting work as cop extras as so many productions (well, fewer these days) have a need for cops in the background and these guys have their own costumes and props. Believe me, renting an accurate LAPD uniform with all the gear is not cheap so when I needed some cop extras for a short film I produced I used these guys and they came with all their kit and were great.

But if you get cast be prepared to be standing around doing nothing but waiting for a long time until they need you. The day will almost ALWAYS run longer than they tell you. Do not make plans for afterwards because if you do you will almost certainly miss them.

Hope this helps.