Most jazz musicians - and maybe even non-musicians - will know just how evocative good four bar intro can be. It starts your foot tapping. It attunes your ear to what is to follow. It is the passport control to a musical journey that can lead anywhere.
You just never know in advance. Routine licks. Something special that just appears from somewhere. "Oh dear. What the hell I am doing.".
André Hodier called jazz - The Sound of Surprise. How right he was. It is this journey into the unknown that makes jazz so special for both the musician and the listener.