A certain degree of planning has to go into an improvisation. I think it’s rare for a musician to improvise something that they have never played before. I have to practice certain aspects of theory in order to improvise. I think the most important part that needs to ben reviewed is the melody. The chord progression is important to play before hand as well. I would already need to know where I’m going to go before I play it. I think if there are musicians out there who can improvise something without having played, planned or thought about it, they are truly remarkable.