It's hard to imagine what the Alien movies would look like without the singular vision of Swiss artist H.R. Giger. But before Giger was hired on to design the film's monsters and worlds in the image of his own Freudian, biomechanical nightmares, Alien director Ridley Scott and screenwriter Dan O'Bannon were struggling to imagine what their extraterrestrial horrorshow would actually look like.
When O'Bannon sent H.R. Giger a letter back in July of 1977 with a list of elements of the screenplay that needed designing for the film, there were still a lot of questions about what various elements in the film--like a xenomorphs, chestbursters, or alien eggs--would actually look like.
Some of O'Bannon's descriptions ended up making it to the screen almost exactly as he described them (the alien egg, which O'Bannon calls a "spore pod"). Others are way off; O'Bannon, for example, suggests that an alien which resembled "an over-sized, deformed baby" might be "sufficiently loathsome," but goes on to say that Giger should "feel free to create [his] own design." Giger did, and thank god for that. Otherwise, Alien might be a very different, and much less scary, film.
Here's O'Bannon's full letter to Giger, courtesy of Lists of Note:
ALIEN
LIST OF ELEMENTS TO BE DESIGNED
EXTERIOR, ANCIENT TEMPLE. Approximately 20 meters tall. Should suggest an ancient, primitive and cruel culture.
INTERIOR, TEMPLE. This is where the Spore Pods are stored. This room is entered through a vertical tunnel in the roof (the normal entrance has long since collapsed). The Spore Pods can be seen ranked around the altar in the center of the room.
SPORE PODS. These are leathery, egg-shaped objects about one meter tall, which contain the larva of the Alien. They have a small "lid" on top which can pop off when a victim approaches.
THE ALIEN, FIRST PHASE. This is a small, possibly octopoidal creature which waits inside the Spore Pod for a victim to approach. When someone touches the Spore Pod, the lid flies off, and the small Alien (First Phase) leaps out and attaches itself to the face of the victim.
THE ALIEN, SECOND PHASE. Once the Alien (First Phase) has attached itself to the face of a victim, it lays eggs in the victim's stomach, and the egg grows into the Alien (Second Phase). This is a small creature which bites its way out of the victim's body.
THE ALIEN, THIRD (MATURE) PHASE. Having left its victim, the Alien promptly grows to man-size, whereupon it is terrifically dangerous. It is very mobile, strong, and capable of tearing a man to pieces. It feeds on human flesh. This creature should be a profane abomination. Our producers have suggested that something resembling an over-sized, deformed baby might be suffieciently (sic) loathsome. In any event, we wish you to feel free to create your own design.