https://www.billboard.com/culture/tv-film/rob-reiner-talks-spinal-tap-sequel-cannes-1235073669/
"The 1984 movie had no script, just a four-page outline. It was almost entirely improvised. Reiner's first cut of the film was seven hours long. Even the jokes they did have planned — like the infamous "these amps goes to 11" scene — were filmed off-the-cuff."
This blows my mind! No script? Seriously that doesn't happen in movieland... Git 'r done Meathead!!!