The Auditors in RM are not allowed to have personality. In fact, the new Death, who is to replace DEATH is either an auditor or a combination of auditors. In the final confrontation between the two one can see this--when the New Death (note no caps--speaks in Italics) says
We will enjoy this.
The new Death advanced. Bill Door backed away.
Yes. The taking of one Death is the same as achieving the end of a billion lesser lives.
LESSER LIVES? THIS IS NOT A GAME!
Later, on the Mountain Top before DEATH meets Azrael, three of the Auditors appear and threaten to inform Azarel. DEATH grabs one by the robe and pulling its empty hood level with his eye sockets says DO YOU KNOW WHY THE PRISONER IN THE TOWER WATCHES THE FLIGHT OF BIRDS?
It said, "Take your hands off me ...oops..." (emphasis added)
Blue flame flared for a moment.
The Auditors, in their natural state are rational, thinking beings, capable of speaking and of acting -- but only together. To claim individuality is to die.
Later, Pratchett more fully explores this part of his concept of the Universe and the way it functions in ToT. But, Trish, what we learn is that the Auditors are only vulnerable when they, by becoming incarnate, begin to have human emotions. Because those not incarnate have been following what happens on Discworld, some of that individuality and emotion has leaked into them. That is what allows the 5 Horsemen to battle them successfully.
And at the end of the battle to restore Time to it's rightful place, Unity and Susan are talking, and Susan is worried about all the auditors on Discworld that they haven't disposed of.
Unity talks about the effect of sleep, which she said she barely survived. The sun is setting. And with the end of the day, the body requires sleep.And Unity says,
"Can you imagine what it is like? For an intellect a billion years old, in a body which is an ape on the back of a rat that grew out of a lizard? Can you imagine what comes out of the dark places, uncontrolled?"
She goes on to say they will die in their dreams.
And even though it is possible for un-incarnate auditors to slip up--those who have bodies, have become "human" and thus have individuality and personality, who have that sense of the dark space behind the eyes--are doomed. It appears to me that Unity has managed to survive only by becoming insane.
I liked your analogy, Trish, to the ultimate IRS or Inland Revenue. I've always thought of them as the Ultimate Librarians who want every book in the Universe in its place on the shelves, the doors locked--and no person (each one writes his book) can change anything.