I learned that one by accident. I used to do open mike standup (really) and would amuse myself by composing bits talking out loud during long commutes. I couldn

I learned that one by accident. I used to do open mike standup (really) and would amuse myself by composing bits talking out loud during long commutes. I couldn't write them down so I would end up composing/practicing a piece 40 or 50 times the week before a show, tuning every little word and intonation. When I got up on stage they rattled off like magic, and I had several words/tones to choose from right in the moment, depending on the mood. The illusion of spontaneity takes a lot of practice.