This is very topical for me. I ran a weekend improvisation course in December and it was amazing what some people who felt antagonistic to playing anything other than chords or tune came out with by the Sunday! Friday sees me recharge my own battery with the 'long pedal' down the Rhône to Grenoble's
annual impro course. It's been running about 30 years. I've only got to a few recent ones, all in the past 6 years. This year there is a dance impro course in parallel - should make the evening sessions fun
Pignol/Milleret teach a fairly strict line based on an understanding of scale modes, and what you can get away with playing 'in' or 'out' against various chords. But in the end you have to come up with a musical line, and 'targeting the 9
th, or whatever only tells you where you are going to end up.
Other teachers on the course such as Jérémie Mignotte (flute) are much more based on singing a line, and then playing what is in your head. Of course both work, and you do need to the technique to do as well. Ray Charles had that 'sussed'