Before I moved to Spain, I was a bit worried hoping I'd be able to get the residencia. In the end, the worry was unjustified... it was so easy. Just print off the form and fill it in before you go, bring your passport and take a copy of your passport down to the policia and they didnt ask me any questions at all. A quick pop nextdoor to the bank to pay the E10 charge and a few mins later popping back to the policia my residencia was confirmed! Sweet. I've read stories since that the Spanish are a bit more lenient in their approach to handing out residencias and I certainly found that the case.

But still, be prepared!