Back in 1977 a friend of mine who owned a small used record store in Cambridge, MA, was approached by an independent film company who were making a fictional film about a small weekly alternative newspaper in Boston. They liked the quaint look of his store and wanted to know if he would be willing to allow them to film in his store - it was a scene in which the music writer for the paper sells promo LP's to the store. To make a long story short, I and several other friends had the opportunity to be paid extras in the film, although our footage ended up on the cutting room floor.
I did stick around to watch the filming of the scene in the store, which involved the owner (my friend, playing himself) and two young actors. One of the actors struck me as somewhat of a horse's ass - full of himself and very "actor-y"; the other was quiet and earnest, and very good in the scene, which typically, was shot several times from several angles. The experience taught me something about how films are made, and interestingly, both of the actors went on to successful careers. They were Jeff Goldblum and Bruno Kirby. The film, btw, was "Between the Lines", which occasionally shows up on TV.