So, on improvisation - I had the honor of playing with Mike Metlay's Different Skies group twice - once in 2007, and once in 2010. The concept of DS is to bring a group of musicians, usually around 20, together in one spot for a week with the goal of giving an hour-and-a-half concert at the end of the week.
Some folks brought worked-out pieces, ready to be assigned; others brought basic "fake book" charts; yet others brought ideas for pieces; and some brought their instruments and a willingness to listen to what was going on to make a piece of it.
In addition to the daily rehearsals (about 8 hours a day, with a break for lunch), evenings after dinner to about 10PM were "all-star jam" time - everyone's faders up, and improvisation with no cues other than listening to what was happening and making a contribution - and of course the recorder was running, capturing it all. The 2010 all-star jams were standouts - Mike was able to pull two full CD's worth of compositions out of the Wednesday and Thursday jams alone, when we'd really gotten to know each other musically. (There were a couple other years with great results, too; I just didn't happen to play then.) There's a recording of the concert, which is fine, but the jams were much better that year, when the wordless communication happened and something unplanned but beautiful, or hair-raising, or mind-expanding would happen.
Mike did it for 10 years, and I only wish I could have gone every year.