I was lucky enough to play quite a bit with the late great Willie King and most of his jobs were in the flash band style.
He would book a show, then get on the phone and call people until he had a band for that job, then everyone shows up at the appointed hour and place. You never knew until you got to the venue who you would be sharing a stage with. Willie had a thick book of contacts and he spread the love so you may play with the same guitarist or drummer all along, but it was rare to play with the same group. Willie had his favorites but the job was the thing and whoever committed first got the gravy.
Willie had a lot of his own songs, but he was also a living encyclopedia of early country and electric blues and about the only warning you got was the name of the song so you had to be quick or be dead because he would call a song and go, and he might not play that song in the same key he played it in the last time you did it with him and he never played it exactly the same way so you needed to have eyes on his left hand when he called the song to see where it started. You learned to notice things like when he was in a solo and looked left, change was coming. When he called a song be watching his feet, one of them would always be patting to whatever tempo was running through his mind when he kicked it off. If you were slow on the uptake you didn't get any more calls from Willie.
"Strictly speaking, there is no certainty, there are only people who are certain." -anon-