One started as a fourth assistant, then moved up to a third, who handed film (8x10) on set and was in charge of equipment checks and dark room supplies. As the top moved on, so did the other positions, once you got to second assistant you were on camera, focusing, cropping and following Dick's framing requests, also as second you were printing all the black and white prints that came out of the studio. One did the printing so that when he was promoted to first (studio manager) assistant he had a full understanding of how Dick wanted things lit.
We all learned from each other and mostly from the handed down system. So me coming in as a second assistant and becoming first within a year and a half was quite rare and exciting.