One problem I had, even after a good thorough soundcheck, is that I raise or lower my my voice for dramatic/comedic effect either by projecting more forcefully, or approaching and backing off from the mic.
The SE then takes it upon himself to raise or lower the volume as he hears fit, thus eliminating the effect I am trying to achieve.
So now, at a festival or show, I'll tell the SE ahead of time that when he hears my voice level rise or fall, not to adjust it, because that's what I want.
Most of them appreciate the information, and during my performance, they can lean back and have a beer without haing to tweak.

Seamus