Back when I was trying to break into show business, I went on a few cattle call auditions with my roommate at the time. One time I was running a little behind and didn't get to the audition center until right at the advertised starting time. The line was already out the door, and wound around a complete New York City block. Everyone in line was dressed just like I was. My roommate stayed in the line, but I went home.
He said the line actually started moving quickly once the audition process started. They all handed over a head shot and acting resume to an assistant at the door, then sat in groups of about 50 while someone walked around them. Some people got a tap on the shoulder, and the rest of the group just walked out the back door. He wasn't tapped, so his day was over.