I once had to "fill in" for my boss while he was away for several months. There was no talk of acting rank, but the subject of "substitution pay" was raised.
My initial reaction was, "I'm not taking a pay cut..!" At the time my boss was a Sqn Ldr Engineer, while I was PA aircrew. However, there is apparently a rule to cover this sort of situation, covering for someone of higher rank but lower pay, and I eventually got a (very) modest pay increase for the extra work and responsibility while was boss was away.