While most free jazz is surprisingly structured (nothing wrong with that), there is a form of improvisation that is approaching total freedom. I'm thinking of artists like Peter Brötzmann, Paal Nilssen-Love, Mats Gustafsson etc. I like this music a lot, and it is not random noise to my ears at least.
In this form of music, I assume that the most important thing is for the musicians to really listen to each other, since not much is prepared up front. But besides that, what is the approach? How much is determined up front? Since different pieces have different sounds and tempo, one can suspect at least some premeditation...