The substitute prepares for usefulness that may arrive with almost no warning.

Some readiness never gets used and still has to be complete. A stand-in learns the whole part, keeps it warm and stays available for a call that may come at an hour’s notice or may never come, and there is no way to prepare partially for that. The work is real whether or not it is needed.

If you rely on somebody’s readiness, acknowledge it in the months when nothing happens. And if you are the one waiting, count the preparation as work you are doing rather than time you are wasting. Staying ready costs something to hold.