Ideally the point is to ask this question before you have them stop doing the job.
"What will happen if this person stopped doing their job?"
"Oh the important monthly reports will stop going out."
"Oh, okay, we better not have them stop doing that then."
Bad news: you have been rejected from consideration for any work related to realtime/life critical systems.
Good news: you may have a future in politics, at least in the county I live in.