The cast and director of the legendary This Is Spinal Tap, the 1984 film that revolutionized the mockumentary movie, reunited for the 2019 Tribeca Film Festival.

Rob Reiner’s improvised opus that starred Christopher Guest, Michael McKean and Harry Shearer as a heavy metal band from England as they headed out on a disappointing concert tour changed the game. Mostly because they created something so true to the music world that, when they released the film 35 years ago, more than a few people didn’t realize it was all made up for the fun of it.

They shared the backstory of the band and movie in this interview with Harry Smith for NBC.

Reiner said his favorite line exemplified the improv genius this group shared throughout filming.

“It’s such a fine line between stupid and clever.”

“It’s how conversations actually happened,” McKean explained. “That’s what was so much fun. Nobody was looking for a punchline at any time. We were living in the scenes.”

It all goes back to an ABC late-night special in 1979 called The T.V. Show, which was a parody of The Midnight Special (the late-night musical variety series that was airing at the time on NBC). They made a 20-minute short film instead of writing a screenplay. They’d written extensive backstories for each band member, as well as the film’s director, but couldn’t figure out a script because they wanted to ad-lib the whole thing. When they showed the short to studio execs, it baffled them because they couldn’t figure out that the band wasn’t real.

Shearer said the studio’s marketing people suggested they add an actual wink in the first minutes of the film to let audiences know it was a joke.

Fortunately, these comedians knew what they were doing.

Such a fine line, indeed.