Q: You've said the best way to come up with chords is by selecting the chord(s) that best support the melody... But, what about if you're accompanying a soloist who is improvising and you don't know what the melody notes are going to be? How would you come up with interesting chord substitutions/changes to support the melody if you don't know where the melody is going?
A: start by doing the most basic thing according to the song form. then as you go, you should get a feel for where they are going, and they will get a feel for where you are going. it gets better and better with experience. don't forget to have fun, stretch out, and relax kind of like you are in a lounge