I thought this analogy was pretty interesting, from a comment from a pretty recent post:
Imagine, a script for every occasion, all kept in several filing cabinets; a secretary sits at a desk nearby, jabbing her fingers away at a typewriter keeping notes and processing the continuous stream of thoughts I'm having that are being used to adapt and write new scripts for me to perform. I might ask her to fetch a script from one of the drawers in the cabinets when I need it, or sometimes I spontaneously do improvised acting, flying script free. I like to improvise especially when it's in my best interests to do so, as prior scripts don't always suit the occasion. Afterwards I sit on the top of my secretary's desk and write a new script using the new material from what was improvised, sipping some hot black coffee and musing on how to better perfect my art of acting.