10 Horror Movies That Made It Up As They Went Along (But Were Awesome Anyway)

The original script for Alien had little resemblance to the finished project. Ripley was a man. The Xenomorph was a conventional looking extra-terrestrial (with eyes). Ripley was meant to die at the end.

Although a lot of these factors were ironed out early on, there was one massive plot hole that didn't get sorted until much later. Even though the Xenomorph looked suitably terrifying, screenwriter Dan O' Bannon couldn't think of a compelling reason why the crew couldn't just shoot the blasted thing. Director Ridley Scott thought having an indestructible monster was too cliched and so needed another explanation why the characters didn't put a bullet in the creature's skull.

After much deliberation, concept artist Ron Cobb suggested the Xenomorph should have acidic blood, meaning the characters couldn't shoot at it without risking themselves being scalded to death.

This teeny-weeny change made the concept of the Alien ten times more terrifying. In every other sci-fi flick, any monster, no matter how powerful, can be obliterated with gunfire.

But not only does this strategy not work in Alien, it's literally the worst thing anyone could possibly do. Not only did this addition solve a plothole, it made an already scary movie far scarier.