The organist and pianist at a former church was really good. He would do his own arrangements of familiar hymns that were great. One time I asked him how long it took him to write the arrangements.
He said years previously he would spend hours and hours over the course of a week or more working on an arrangement. But then he learned the basic concepts of jazz and he said he now could do it in an afternoon.
I went to a concert with Phil Keaggy and he had a bunch of musicians with him that he had just sort of thrown together for some shows on the west coast. Someone from the audience requested a song.
“Well - just me (he plays guitar) and the drummer know that one of mine. But the others can chime in when they get a feel for it.”
It was unbelievable how the others (4 or 5 others?) figured it out. I have no idea if it sounded like the original song, but it sounded great.
I recall one gal started playing one instrument (a violin?) and after a short time set it down and picked up a flute to play instead. I guess she figured the flute was more compatible to the song.