In the last scene of Alien, Ripley escapes in a shuttle craft. She screams when she sees that the alien is onboard. Why didn't the alien kill her immediately when she boarded the shuttle craft? Is it sleeping? How deaf would a 'perfect' species have to be to not hear someone scream at the top of her lungs right next to you?
The alien was, iirc, huddled on its side in a small alcove. It may have actually been sleeping, or in a minor hibernation state, as we saw in the sequel Aliens that the aliens stay motionless in their natural environment, invisible to both thermal and motion sensors, until roused.
Another possibility is that it was preparing to molt. We had already seen shed skin from it as it rapidly grew. Even though it appeared to be quite large, it is possible that it was resting in preparation for a (final?) molt.
One key reason the alien doesn't kill Ripley immediately is explained by a rather famous deleted scene from the theatrical release. The scene shows Ripley, after Parker and Lambert are killed, finding a still-alive captain Dallas, who they all had assumed was killed earlier by the alien. In fact, Dallas had been cocooned by the alien, along with Brett, and they are each gradually mutating into one of the original eggs that birthed the facehugger.
This illustration of the original plan for the alien life-cycle shows that the alien doesn't need to kill everyone, and in fact shouldn't have the need to kill everyone in order to adequately propagate the species. If the alien were a blood-thirsty, mindless killing machine, this life-cycle would be hindered. The same can be said for aliens in the second movie, who also cocoon victims, but in that case they cocoon them so they can be "facehuggered" later from eggs laid by the Queen. So, whether or not this deleted scene is considered, as a rule an alien wouldn't be expected to kill every victim so that it might keep the species going.
However, the alien will need to kill for food or if threatened. It probably wasn't hungry as it had grown to full size and had killed Parker and Lambert just prior. And consider just how easily the alien had dispatched the rest of the crew, even the ones with flame-throwers. Those soft, pink creatures with no innate weapons or defenses and relatively poor senses and reflexes are hardly a cause for worry from the aliens point of view. Add in the fact that the escape pod was a very compact space (i.e. advantage alien) and the alien likely didn't feel that Ripley was even a remote threat to it. It could conserve its energy and take its time. Once she started blasting it with gases, it changed its mind.
I have three theories on this.
Theory one: after Ripley started the self destruct sequence the ship went berserk with warnings. Perhaps the Alien realized that it's environment had changed for the worse and wasn't entirely sure what it needed to do to survive. The logical thing to do would be to watch how it's surviving quarry behaves. In fact, this is exactly what it does; it lies low, follows Ripley and observes. Killing her just because she screamed would not have aided in it's survival; keeping her alive and riding shotgun to safety would.
A second possible explanation. Let's say for arguments sake that the Alien, like ourselves is hardwired for survival and propagation. It was young, hungry, enthusiastic and confused when it killed the other crew members but when it matured it must of realized that Ripley was the only living thing in it's Universe that could host new Aliens. It was probably more interested in keeping her alive for cocooning/implanting than ripping her head off.
My third explanation is that the creature didn't hear her or didn't understand her. Consider, in Alien Resurrection the imprisoned Aliens communicate with each other to co-ordinate their breakout but they don't use sounds (possible pheromones or some kind of telepathy). It is possible that they do not have any kind of auditory senses. Even if they do, the only sounds they make themselves are that horrible screaming so hearing it from a completely unknown life form may not have been a clue that it had been discovered.
This is not a “huge question left unanswered”. The only question that needs answering is why you guys think that just because an Alien isn’t stuffing victims into its mouth like it’s a hotdog-eating competition, something must bewrong with it. It’s broken, mommy! Jeez, can’t an Alien take five once in a while? It just ate. The crew.
Okay, look. The Alien is a species that has managed to spread to different planets without developing tools or technology. It does this by acting as a parasite on other species. In the sequels, Aliens ride elevators and cut power lines. Is it really such a stretch to imagine that an adult Alien might be smart enough to conserve its host species until it is needed? Lions follow herds across the plains, killing them to feed themselves or their cubs, but never for sport.
The Alien followed Ripley and her cat onto the shuttle, because that’s what a predatory organism does when a herd of prey migrates. It follows. Casually. No rush. If you drop two mice into a snake’s cage, the snake might eat one, but will always save the other till later, not even looking at it. This does not mean the snake is incapacitated or “defenseless”, nor does it mean the snake hasn’t noticed the mouse.
I’m not saying the Alien is a reptile specifically, but think about it: It would be an extraordinary coincidence if Ripley’s ship stumbled onto the crashed spacecraft containing a nest of alien eggs soon after it crashed. Far more likely the crashed spacecraft, and the eggs, had been there for a very long time, centuries even. Yet the eggs were still active. This suggests the sort of energy-conserving rest mode we see in many reptiles, and bugs like fleas.
To sum up, the Alien was resting, saving Ripley and her cat for later, biding its time until another herd of human food comes along, from wherever herds of humans come from in this little metal universe. Then the alien got gassed, and it decided (correctly) Ripley probably had something to do with it.
I guess you are talking about the first movie, Alien.
This Alien-creature is not old. It gets "born" after eating its way out of her coworker, and is only a few hours old. It gets to full size in about 15 minutes.
Any young creature who feels like it owns the universe will at some point think it is invincible. I think that this is what happened to the Alien. It felt it had killed all the others fast and "easy", and when the last person screams it starts to take its time, for the fun of it. Like a cat who isnt hungry starts to play with the mouse before killing it.
I always assumed that the reason the Alien didn't immediately jump out and eat Ripley was because either:
It could tell that she was the last human, and would be required for egg placement later.
In maturing so quickly, it's mental comprehension had also developed, and the Alien picked up on the fact, perhaps from Ripley's behavior, that the area they were in was the only safe place during a tragic event, and chose to stay hidden. Scared animals don't tend to hunt until danger has passed.
And a less probable explanation that I heard was interesting - The Alien might not be fond of spoiled food, and was leaving the helpless pink human animal alive until the Alien was hungry again. As several have pointed out, it had just eaten quite a bit earlier that day.
The alien was coming to the end of its life cycle, when Ripley happened to disturb it. It was slow to attack because it was dying. This theory is supported by an older version of the ALIEN script where Ash reveals that the alien had made a nest and ensured the continutation of its species (cocooned Dallas and transformed Brett into an egg) at which time the alien itself would approach the end of its lifecycle; curl up and die.
No no no! The alien doesn't even notice her! It is ill or something. It is completely shocked when Ripley turns on the high pressure jet on top of it and doesn't wake up when she tries the first two jets. It is kind of moaning in its sleep when she sees it and screams. I think it's either knackered or naturally dying as it has a very short life cycle and has (maybe) impregnated Lambert therefore completing its purpose.
It might be really tired, because the reactions of the "chestburster" come in after it turns into a "xenomorph drone" and this is the first time the "chestburster" bursts out of the rib cage of our species. Also another reason it didn't kill Ripley is because it was evolving into a xenomorph warrior and evolving into another stage is tiring and exhausting for xenomorph.
The Alien had impregnated the cat. That done, it wanted to make sure that the cat would safely reach a population of other living creatures, be it humans or animals or any other species. It figured out that the only sentient creature that could do that (i.e. navigate the ship and reach home safely) is Ripley. So it wanted her to stay alive until the ship reached its destination and another Alien companion would emerge from the cat's body. The two of them would then have greater chances of impregnating other creatures, starting a new colony of aliens. I believe also that the Alien was in a state of hibernation, in which its senses, movements and general aggression might be muffled.