I didn't intend to do any theater for the rest of this year. Told my new job that. Didn't expect to...until yesterday evening, when I got a frantic email.
Fortunately someone else was bringing gear, and there were two people setting up when I got there. I was mentally prepared for it to be anything from "come back at 9 to help us pack up" to A2 to...well, that was a good thing, because I was FOH. Simple show, though. Three-piece combo, lead vocal, guest tabla, a couple of choir bits.
Unfortunately the mains were set up right behind the band. They liked it -- meant they didn't need monitors -- but meant I couldn't do overhead on the drums or as much as I'd like with piano. Baby grand anyhow, which with a few noted exceptions always sound bad and are impossible to mic. No time for sound check, no rehearsal, no idea what the show was going to entail, so I had to stick with whatever I gambled with in setup and hope it worked.
EV on kick and snare (don't remember the precise models at moment). 57 on the bass cab, and I was glad I had it. Shure PX (sigh) on lead vocal. And tried out my Karma silver bullet on piano -- it was on full stick, so under the lid and as far as it could get from the strings. Wolf tones and nasty harmonics, plus incipient low-end feedback, but possibly better results than I would have gotten with any of my other mics (the Karma is like that...weird, almost indescribable mic and sometimes that's the right choice).
And that's the name of the game some times. Take your best guess from experience, stay calm, react to the changes, and deal with it ahead of time that even if the sound doesn't suck you aren't going to be satisfied with it.