In every show you watch or movie you see, there are tens, hundreds, sometimes thousands of extras -- people that are in the background of shots: making copies at an office, driving past, marching in an army, etc. In fact, out here it is often called "background" work. So, in a city like L.A. where so many shows/films are produced, it's no wonder that there is A TON of this background work.
Now, most "real" actors (out here and back in Chicago) look down on extra work, since you're not really acting, you're just sitting or standing in the background -- and it's never put on an acting resume. It's also a strange dynamic within "background actor" circles. There are many folks who make a career out of doing extra work, which I can't really understand, because it takes some loooong days of mostly just sitting around in a room called "extras holding,"and it is not creatively fulfilling (though many career extras like to pass the time telling each other stories ad nauseum about rubbing shoulders with celebrities...)
But, I've done a bit in the past and I'm doing a little out here. There are two reasons: 1) it's always good experience to be on set, to see how things are done, watch the director and actors at work and 2) it pays. And in these lean times, that's the real kicker!
I've got an interesting project coming up. I'll be working several days over the next couple weeks. If and when the project is released, I'll be sure to let you know, and maybe you'll catch a glimpse of my mug passing by in the background ;)