Solid bass players who are very, very good at big, fat, long notes are de rigeur. It’s way harder than I thought.
Correct! It sounds simple, but hard to not anticipate, synocopate and do the lead-in thing if you are programmed to most rock bass styles. Its actually hard to just stay dead on the beat, and move gracefully from change to change if you aren't used to it. The deal is, you and the drummer are creating this vicious pulse that forces folks to feel and move. The singer and the lead instruments do the syncopation and the anticipated feel. You have to have the beat, the off-beat, and then everything else. That's country.
I don't know that you have to switch, as much as just add country to your set list. I haven't really played that many crowds that weren't just generic. Most gigs I've played, its a mix. Some folks dress like they are at the beach, some are wearing black dresses, some might have biker gear on stepping in from a ride, some plaid, and so on. I guess a lot of bands don't get that folks will listen to anything that's good. I see bands mix Bruno Mars, Eric Church, Allman Bros, Monkees, Jason Aldean, Prince, Hank Williams, Styx, Black Crowes, Manhattans with success. And the crowd doesn't really notice that its not all country, all rock, all R&B, etc. And folks want to dance, but they won't if you don't play dance music. They want to sing (after a few beers), but they won't if you don't do something familiar. I think the rock vs country vs pop is more important to the band members and their identity, than to the crowds.
And ditto on the volume thing. Country is LOUD, but not brain squashing loud. Rock doesn't have to be offensively loud, but unfortunately, a lot of unrestrained folks with Marshalls have paved the way to the conventional rock band what runs everyone off midway thru the first set. "I gotta get my sound, dude" or "rock is loud, man. If its too loud...".