Deputize

I want Tom to deputize for me.

The sheriff decided to deputize some civilians.

I deputize you to act for me while I'm away.

Gringoire, on the other hand, stuck with equal firmness to the fact that he was deputizing for a brother officer who was sick — so sick that he had mislaid his orders.

What he wanted was someone who could really deputize for him when he was away on business – clearly not the job for a secretary but, because of years of experience and a willingness to take responsibility, one that the previous holder of the post had taken on.

It is commonly used in the theatre where an understudy essentially deputizes for an indisposed actor.