During college I worked at the Gap. We were a big Gap, but one not in a mall so we didn't have any heavy foot traffic so it was pretty common for TV people shooting in the area to come in all the time for wardrobe stuff.
Being in North Jersey, 2 big shows that filmed locally were "Ed" and "The Sopranos"
The guys from Ed used to come in mid morning on weekdays and do their thing, so I would end up talking to them a lot. One time we got to talking and I brought up how I lived right down the street from the common street they used to shoot on and I walk down on Sunday mornings and watch them film. So they told me to I should always come down even earlier and talk to them on scene when they were preparing the shoot, saying they could always use some extra people around.
I don't know if I was ever on screen shown walking in the background though.
The Sopranos thing was a different story, since I refused to watch the show, I have no idea if I was ever seen.
I was also in a ton of Sports Illustrated for Kids commercials when I was in grammer school. They loved the look of our field and our uniforms so they came all the time during our soccer practices to shoot their commercials. I have them all on tapes somewhere around here, but it's been so long.