It isn't a bad job, but it is tense. Like driving, you can't take a break and you can't take your eyes off the road/the stage for more than a moment.
For stage musicals, that is. For live music, depending on the show, there are times I can get up and walk around (to listen from different vantage points) and even take a quick break. A potty break even -- IF you have someone to watch the faders for you, because howling feedback can start in only a few seconds.
For most of a stage musical, I've got my eyes on the stage, the master faders in my right hand, and the script open on my left. I am of two minds whether it is sensible to nuance faders. Sometimes it seems the only way to get the dialog right is to ride it a little. Certainly, when you have people shouting, putting on hats, or doing a faces-together duet you have to be ready to duck one or more microphones. And the dynamic range between speaking and full-on song is also too great to get by with a single setting. But I am not convinced that riding the fader up and down constantly, breathing with the dynamics of the song and scene, is actually necessary or even helpful.
I also try to avoid playing with the settings too much once the show is opened. Voices change from day to day, the air quality changes, there are more people in the audience to suck up sound, an actor has a sore throat or tries something different...and it seems inevitable that the microphone will end up in a different spot now and then. But there's a difference between making an adjustment at the top of the show to get it back to where it was, and tweaking the EQ and compressor settings every other song, trying to nuance out the perfect compromise.
For the latter, I try to keep in mind what The Rat said about tweaking; "Imagine there is a camera on you and after the show you are going to be asked by a bunch of students why you made every change you made during the show." So think twice, adjust once. Don't tweak it unless you are really sure it needed tweaking.
The thing of it is, besides being trapped at the board -- you can't leave to get a drink of water, you can't even close your eyes for more than a few lines of dialog -- the tension is a killer. You are like the last person with their hands on the wheel before a two-car collision. All the errors have already been made -- in your hands now is the final chance to save something. And when, on the other hand, the show is going well, in your hands is the potential for a single slip to wreck a moment, or a song. All you have to do is stab at the wrong button out of the 200-odd ones spread out before you, dimly, in the darkness of the FOH position. Or twist 5 degrees instead of the two degrees you meant to on an important knob.
The audience probably doesn't hear most of your mistakes, but they very much hear the accidents you weren't fast enough to catch; the microphone that dies, the actor who touches their microphone, the bass player that unplugs without warning anyone. So for all that nuancing and tweaking, your final tally is less how good it was, and more how FEW bad things happened on THAT particular night. It is not a good place for people who stay up late kicking themselves for blowing a quick duck of a microphone they SHOULD have remembered happens on that line of dialog, or for being there when a dying microphone decided to let loose like a cannon shot over the sound system.
Partadoxically, of course, you go into every night incredibly suspicious of everything; checking everything, worrying about everything. But you have to learn how to put that behind you after the performance.
Me, I like to play a little. I like to pull down after songs. I like to use different levels of reverb and different reverb algorithms for different kinds of songs. I like to follow the dynamics of a song and ramp up not just the vocal master but the band reinforcement master over the climax. And I like to throw in other little bits; like starting a mic soft when a character enters from off-stage, and fading them up as they walk on.
So I have a script, but it only has the programmed cues on it (the memorized scenes that hold most of the basic microphones in a particular song or scene). And maybe sound cues, if I am running those as well. It is mostly memory that I use for all the bits where I need to duck a mic or ride it a little (because that actor has always read that line in a soft not-a-stage-whisper whisper). By the end of the run, I'll sometimes forget to turn the pages of my script for two or three scenes at a time.
And the rest is ears and instinct. Watching the show, being in the moment -- but also being conscious all the time of "real" sound (as opposed to the perception of sound) -- watching that your levels don't creep up on, adjusting for how what you hear is different from what the audience hears. It is very much like performing in the orchestra pit, that kind of focus.