The substitute practices for a moment they cannot schedule.

A substitute prepares for a moment that has no date. All the training is aimed at an event that may come tonight, in three months, or never, and there is no way to time the readiness. Preparing for the unschedulable is a distinct and difficult discipline.

More of life requires it than we acknowledge: the emergency, the opportunity, the conversation you will need to have when it finally arrives. You cannot rehearse on a timetable, so the only workable approach is to keep the readiness at a level you can sustain indefinitely. Preparation that exhausts you will not be there on the night it is needed.