FWIW...
In July, I purchased e-tickets for AGP-MAD-AMS and used the machine @ Malaga to check-in a day early (about 18 hours or so). My vague recollection is that I had to insert the credit card used for the on-line purchase in order to obtain boarding passes from the machine. When passing through the regular check-in line, the agent did not ask for the credit card.

I just purchased another e-ticket but this time it's for my sister-in-law using her boyfriend's credit card. (long story - not for FT...) Based on Viajero's advice, I will forewarn her to ensure that the boyfriend is present (with card) when they check-in at BCN.