Cast and crew members laid out special effects techniques they used during the film’s production in the commentary track for “Alien” when the film’s director’s cut was first released on DVD in 2003.
In order to achieve the icky, organic look of the facehugger, Ridley Scott employed some actual organic compounds. The egg would open via a series of metal, mechanical servos, but inside the egg was a slurry of amniotic esoterica made of actual animal entrails. Scott extended his own gloved hands into the egg from an aperture beneath and would wiggle his fingers to make the viscera look alive.
The facehugger itself was initially intended to be a sickly green color, but screenwriter Dan O’Bannon was able to sneak a peek of the creature model while it was still being built and preferred the bone color. This seemed like a wise choice. The creature was coated in slime, a common ingredient in “Alien” films. It’s a well-worn piece of trivia that star Sigourney Weaver suffered allergy attacks on the set of “Alien,” suspecting the cat as the culprit. She later learned it was not the cat, but the glycerin compound sprayed on her face to simulate sweat. Slimes and sprays and compounds were everywhere on the monsters. According to Tom Woodruff Jr., who played the alien in David Fincher’s “Alien 3,” the slime dripping down the creature was the same material used to thicken milkshakes at fast food restaurants.