I Guess The Last Post Was Right

Just got back from a 15 hour day as Script Supervisor (I’m beginning to toss the anonymity to the wind). We were shooting in a food store while they were still open for business, so we had to work around the patrons, which wasn’t too bad. However, when one woman saw that a movie was being shot, she leaned over to a stranger and said “lights, camera, action,” followed by a grin. It’s the film set catch phrase, but why?