Aliens are tough.
If you want scary aliens in your movie, you have only two real options: human-scary aliens, and non-human-scary aliens.
Human-scary almost never works. Human-scary aliens look basically like humans. They walk like humans. They have two legs, two arms, two eyes in their one head.
Human-scary aliens are rarely scary. Or, rather, if they are scary, they are only scary the way that humans can be scary. Take the aliens in M. Night Shyamalan’s Signs.
From ‘Signs’. Just a big-headed guy: not scary.
They weren’t scary, unless they were creeping through cornfields, or leaping out from a dark corner, or grabbing at you suddenly from under a door. But a housecat can startle you if it sneaks up on you – that doesn’t make it scary. These aliens aren’t frightening, or even particularly interesting, to look at. That’s why they are almost never shown straight on, and, when they are, they are in shadow, or blurry. Even Predator, with his far-out face, basically looks and walks like a muscular guy.
Predator. Gnarly, but not scary.
The only way to make human-scary aliens actually scary is to bend their human forms in some way, like the broken-human, many-legged Thing (from the the 2011 remake), which scrabbles around like a crab.
The 2011 Thing. This would get your attention.
Contrast the ‘Signs’ aliens with the Alien aliens. The ‘Alien’ aliens are the best non-human-scary aliens in all cinema. The Alien is bizarre and creepy, fascinating to look at when it stands still, let alone when it moves (although it does move bipedally and have arms, it isn’t humanoid).
The Alien was designed by H.R. Giger, which gives one a sense of how much work and genius it takes to design a really scary alien.
In the opinion of this author, the Alien has never been matched. However, CGI has created some interesting runners up. However, most of them are, like Alien, insectoid. For example, take a look the skittering parasites from Cloverfield (which owe their form in part to the bugs from Starship Troopers, which, though the movie is funny, are a convincing menace).
‘Starship Trooper’ Bug.
Which is why it made for a nice change to encounter the wing-dinging alien Mimics in the ‘Edge of Tomorrow’. These fast-moving and tentacled aliens owe more to octopi than to insects, although even they owe something to Giger, with their semi-mechanical steampunkish tentacles. And while their faces were, of course, disappointingly human, they are the best aliens seen in a long while.
A still doesn’t really do the Mimic justice; their most affecting quality is their movement. They are very fast, and move non-linearly, which makes them difficult for a human eye to follow.
And that is exactly the point of aliens: they aren’t like us. They shouldn’t look like us, think like us, or move like us, and the more unlike us they are, the better they are able to do the job of scaring us.
Anyone who enjoyed the ‘Alien’ movies even a little should take a look at H.R. Giger’s printed work. The Necromonicon is a good place to start.
‘The Edge of Tomorrow’ is based on the novel All You Need Is Kill, by Hiroshi Sakurazaka.
‘Starship Troopers’ is also based on a novel, Starship Troopers, by Robert Heinlein.