Focus is the second most important responsibility of the First Camera Assistant, second only to assembly and maintenance of the camera itself.
The Camera Assistant must understand not only the basic technical theory of focus, but also understand the creative choices of focus-sorting out what you want to keep in focus, what you do NOT want to be in focus, and what you don't really care about.
There is a certain degree of luck in being able to carry out your focus plan for a particular shot. Experience and skill help, of course, but the real secret is deciding on a focus plan for that shot in the first place.
What is to be the plane of sharpest focus? How, when and why will that plane of focus change during the shot? If ...