..but give them what they want, not what they say.
If you are professional, you do the job they hired you for. Even if the client is screaming at you, even if the show stinks, you sit there and you finish the job you contracted on.
But I've worked with more than a few people who think the job stops there. That if the client is an idiot and wants them to turn off all the lights in the middle of the act, turn the subwoofer to eleven and turn all the wedges backwards...then you just do it, and rationalize the horrible lighting and sound by knowing you did exactly what was asked of you.
Well, we are not golems. This kind of petty exact-letter-of-the-orders crap is not professionalism. Find a way to get the client what they really want; the kind of look, the kind of sound, and talk them into how the equipment has to be set up to do it -- even if you have to go behind their backs to do it.
Just remember; they have the last say. Their vision is what belongs on stage, not ours. If they were after the look of a blacklight effect, then we should get as close as possible, but if their artistic goal was actually to kill the lights on the performers and leave the audience puzzled, then we should respect that.
And they might know what they are doing. We are not wizards. We don't know everything, and we can have off days, too. So before you nod and smile and pretend to turn the guitar up without actually touching a fader, take a long hard look at what is actually going through the sound system. And maybe even try nudging that fader. Because it could be the client has a better idea of what that band on that day really needs.
Sure, it is harder to fight for a good show. Sure, it is attractive to take your anger at being yelled at out on the client by sitting there in your self-righteous sulk and letting the unhelmed ship plunge directly towards the pier.
But at the end of the night, the paying audience, the gods of theater, your own artistic growth, and yes even the annoying client; they all deserve better from you.
And that's professionalism.
(No; the show I just concluded was pretty much a pleasant experience. And the rest of the tech staff mouthed off a lot backstage, but they still put out the best performance they could manage. But they reminded me of how many times I have seen techs cross that line. And how many times I've let myself cross it, as well.)