The Beatles album and film Let It Be (yes it was also a movie, although they never show it anywhere these days due to copyright problems) began life as Get Back - a filmed documentary of the band rehearsing for a proposed live performance.
The live performance turned out to be an unplanned and unannounced concert on the rooftop of Apple Records in central London.
The songs performed made it onto the eventual album Let It Be more or less as heard live, and you can see the concert here (by clicking below) as it was depicted at the end of the movie...
Part One
Part Two
Part Three
The reactions from the people on the streets are priceless, and somewhat touching.