Yesterday I was reding about movies screenplays and found some analogies with writing requirements . The screenplay must be readable, because filmmakers need to understand - what they are talking about, - who will be there, - how much it will costs - etc. It is usually roughly divided in three parts: the set-up, the development, the epilogue (eg. 30, 60, 30 pages) Many things change before and during shootings: dialogues change, sometimes you shoot a scene at night instead of during the day, etc. so.... "If you don't like changes, then go write poetry". This one rings a bell, doesn't it ? Also, it will be read by the actors, who will interpret it and change it in some ways.
Ever worked a lot and ended up realizing that the customer wants it another way?