This is Spinal Tap 

Rob Reiner’s hit 1980s movie This Is Spinal Tap follows the lives of rock metal band. Its success introduced the entire “mockumentary” concept, paving the way for movies and shows to follow suit. 

The three lead actors, Michael McKean, Christopher Guest, and Harry Shearer were so excellent at ad-libbing that many of their improvised lines stayed in the movie. They even each got writing credits at the end of the movie. Of course, one of the most famous of those lines was, “It’s such a fine line between stupid and clever.” That pretty much says it all.