Christopher Guest, Michael McKean, Harry Shearer, Rob Reiner

This is Spinal Tap is Christopher Guest, Michael McKean, Harry Shearer, and Rob Reiner's baby.

At this point, it's a baby that's several decades old, curses a lot, and has an overabundance of chest hair.

But we still love it as only a mother can.

All three actors playing band members were musicians themselves at some point, and they'd seen the self-important heavy metal bands and watched rock documentaries. Guest had once witnessed an inane conversation between a drugged out rocker and his manager that inspired him to create the character of Nigel Tufnel. (Source)

In 1978, Guest, McKean, Shearer, and director Reiner had been working on a comedy special for ABC. One of their sketches was a rock and roll parody—a first look at Spinal Tap doing a song called "Rock and Roll Nightmare." They realized a good thing when they saw it and decided to see if they could turn the concept into a full-length movie that aimed to be a completely realistic satire.

Shearer told TCM that the initial impulse to make the movie "was our mutual shared frustration when we saw all of these rock 'n' roll movies, and they kept getting it wrong. People were supposedly playing guitars with their fingers in impossible positions, given what we were hearing. Forty million people in this country have taken guitar lessons, so why would you insult the audience quite that gratuitously if you don't have to?" (source).

Anyway, "writer" is a term that needs a bit more elaboration in the case of Spinal Tap. See, most of the movie was improvised. Which is part of the reason there are so many absolutely bonkers, totally zany lines we can't quite imagine someone sitting down at a desk and jotting down.

"Dusting for vomit" isn't a concept that occurs to you unless you're really in the moment.

That said, each scene was at least roughly outlined, and it's these four who did the bulk of the work. It's worth noting though that they did try to get the Writers Guild of America to give every single performer in the movie a writing credit. It didn't work, but it's a pretty awesome idea. That really would have bulked up some people's IMDb credits.

Typically, a script is shopped around until someone agrees to buy and produce it. Once the writers realized that a script would be totally impossible and beside the point—that the dialogue begged for improvisation—they made a 20-minute trailer instead. Almost every major film studio rejected the project. Christopher Guest said, "People said, 'No, this is—I don't even know that this is'" (source).

The demo finally caught the eye of producer Norman Lear, the television producing genius behind All in the Family, the groundbreaking 1970s TV phenomenon that made a star out of Rob Reiner. Lear was the only guy willing to take the risk of sinking millions of bucks into an improvised fake rock documentary.

We're grateful.