That’s really what blocking is all about. You set places for the camera and talent to be (set distance) and the focus should be set accordingly. If people are moving (or camera is moving) you simply pull the focus from one block to the next (like key frames).
There are cases where the AC needs to manually (with eyes) focus, and yeah, those are very skilled individuals. I was looking for a video of Chivo’s Focus guy working while Chico worked the frame from the Revenant, but I can’t find it.