Science Fiction; Horror; Thriller
What a Piece of Junk!
If Luke Skywalker had known how bad it could be, he wouldn't have been so hard on the Millennium Falcon. Compared to the Nostromo, the Millennium Falcon is a luxury cruise. It comes with 3-D monster chess after all.
Alien is certainly a card-carrying member of the science fiction genre, but it uses sci-fi elements in an unfamiliar way. Gone is the romance of traveling the universe, seeking new life, new civilizations and one-night stands with green-skinned space babes. Gone too is the glitz and glamour of adventure and finding one's self among the stars.
In its place, Alien had created a pretty humdrum version of science fiction. The crew of the Nostromo contains your average guys and gals doing a blue collar job for that sweet, sweet paycheck.
Associate Producer Ivor Powell put it nicely when he said:
The truck drivers in space analogy is, I think, a pretty good one. And that appealed to Ridley because the thing that didn't appeal to him as an art director was the shiny Flash Gordon surfaces. He wanted the thing to be just like a battered old truck where there was bits of tape over cracks in the chairs they sat in and things like that. (Source)
Fighting off an alien force while helming the flagship of the Federation's fleet with all its alien killing bells and whistles? That's exciting. Fighting off an alien in a ship that is only working thanks to a jury-rigged engine and some wishful thinking? That's just nerve-racking.
And nerve-racking perfectly segues into the next of Alien's genres.
Guess Who: Genre Edition
Even for genre buffs, the difference between a thriller and a horror film can be difficult to discern. The two have a lot in common and are closely related on genre genealogy. In fact, several films can be labeled as both at the same time, like Alfred Hitchcock's Psycho (1960).
With that said, let's see if Alien is a thriller or a horror film. But first, we're going to need some working definitions.
According to Filmsite.org, thriller films are "known to promote intense excitement, suspense, a high level of anticipation, ultra-heightened expectation, uncertainty, anxiety, and nerve-wracking tension" (source). Thankfully, you don't have to feel all that at once because thrillers would be exhausting affairs.
Going back to Filmsite.org, "[h]orror films are unsettling films designed to frighten and panic, cause dread and alarm, and to invoke our hidden worst fears, often in a terrifying, shocking, finale" (source). Like our thriller definition above, it's not perfect, but it'll do: horror wants to make you scream; thrillers want to get your heart rate up.
Before and After
In light of this, we want to say that Alien is both a thriller and a horror film. Taking it further, we might suggest that the first half of the movie is a thriller while the second half of the movie is straight-up horror.
From the moment the crew wakes up to the point that the alien dramatically exits Kane's chest, we're on the edge of our seats—but we're not exactly screaming in horror. Instead, we have a lot of questions.
We wonder what the signal is, who sent it and why. We want to know what decisions these characters will make given these unusual circumstances, and we're nervous when they explore the obviously foreign and hostile environment of LV-426. We worry whether they'll complete the repairs in time. When the facehugger gives Kane history's least enjoyable French kiss—and that's saying something—we're anxious to discover why the creature is keeping him alive and what it plans to do next. But we're not exactly feeling "panic" and "alarm."
That doesn't set in until the chestburster uses Kane's chest cavity like an express lane. From that point, alarm and dread ratchet up as the beast picks off the crew one at a time.
Now we're bloody knees deep in a classic horror setup, one you're no doubt familiar with even if you have but a passing relationship with horror films. Scream, Halloween, Texas Chainsaw Massacre, A Nightmare on Elm Street, I Know What You Did Last Summer, they all pick off their monster-bait one at a time which gore that creates effects both "terrifying" and "shocking."
The take-home point here is that genres are hardly ever either/or propositions. They exist more on a spectrum of tropes, themes, settings and potential emotional output from the audience. After all, what might be thrilling for us might be horrifying for you; a scene that leaves us anxious with expectation might leave you a terrified mess—or heading out to get more popcorn, waiting for things to get really good.