When the extraterrestrials make first contact, what will they look like? It’s a question pondered by everyone from science fiction writers to theoretical biologists.

You could stock an interplanetary zoo with the creatures created in comics to straight science books, but the directors and special-effects gurus who bring aliens to life in movies and television shows probably face the most difficulty when it comes to tackling the subject.

They must envision the unknown and make it knowable, and their stories can live or die on the strength of their creatures.

When they do it right, with the perfect mix of Hollywood ingenuity and imagination, they can beam you out of this world.

Try to remember the first time you saw those dripping teeth-within-teeth in Alien, your first peek through Roddy Piper’s magic shades in They Live or your first fleeting glimpses of the Cloverfield monster (and its freakish parasites). Even the Horta, the cheesy-looking subterranean blob scuttling through Star Trek episode “The Devil in the Dark,” probably left an impression.

The latest alien to pop into theaters is Paul, the CGI star of Paul, which opens Friday. The excess-loving extraterrestrial (voiced by Seth Rogen) cracks jokes, moons people and swears like a space sailor. Somewhere between the skinny, bulbous-headed extraterrestrials of Close Encounters of the Third Kind and Geico’s smooth-talking gecko, Paul is a realistic-looking CGI alien who seems almost real, like a Pixar product grafted into a live-action film.

(Spoiler alert: Plot points for Paul and Attack the Block follow.)

An Area 51 escapee, he’s the perfect animated foil to Simon Pegg and Nick Frost’s geeky Comic-Con fanboys, who hook up with Paul in the desert. While the pint-size spaceman’s foul mouth earns the film an R rating (OK, maybe that resulted from Kristen Wiig’s epic eruption of swears), there’s not much else that’s shocking about Greg Mottola’s movie, which made its North American premiere this week at the South by Southwest film festival, or about its CGI star. Ultimately, Paul boils down to a vaguely charming string of nerd in-jokes.

Frost said filming the movie, then waiting more than a year to see the finished alien, caused some trepidation.

“It’s like having a colonoscopy at the world’s shittiest hospital and having to wait 18 months for the results, you know, because if he’s bad, we’re fucked immediately,” he told The Playlist.

Pegg added that “the last 5 percent [of the digital work] weirdly enough were the most significant. They start bringing in how the light moves across the surface of his eyes and how his carotid artery flexes and how his diaphragm moves.”

There’s no doubt that Paul is a slickly animated space slacker, but he looks like dozens of bulbous-headed Greys we’ve seen before. The most interesting part about him, aside from the constant patter and punch lines, is his eyes.

But the key to any truly superlative creature design is to surprise us and make us marvel at a new vision made real. Too normal, and we’re not impressed. Too dorky, and we’re laughing when we should be gasping.

That’s the genius of the aliens in Attack the Block. The sci-fi action/comedy, which made its buzzy world at this year’s SXSW, packs a one-two alien sucker punch that leads with the familiar, then K.O.s us with something completely unexpected.

When a meteorite crashes in a dodgy South London neighborhood in first-time director Joe Cornish’s wildly entertaining movie, a gang of young toughs discovers a vicious humanoid creature with big teeth and a nasty disposition. Defending their turf, the teens quickly kill the chupacabra-like critter and parade its rotting corpse through the streets.

More aliens inevitably follow. But thanks to a deft maneuver by the filmmakers, the others that arrive look nothing like the first — in fact, they look completely new and original.

I won’t spoil the movie’s big reveal (you can see glimpses of the beasts in the trailer below), but suffice it to say that Attack the Block’s second wave of invaders look like few things we’ve seen onscreen. They are remarkably simple, Cornish said after his movie’s SXSW screening, and rely on “basically a rotoscope technique, because we couldn’t afford CGI monsters.”

Working with effects house Spectral Motion, the filmmakers cooked up a creature suit that was inhabited by Terry Notary. Cornish called him “the best quadruped runner in the world.”

“We figured, get a guy in a suit, shoot it and then rub out the reflections,” Cornish said, describing his eye-catching and wholly original creature design. “So, I thought of it like a graphic novel.”

Turns out Attack the Block’s aliens were inspired by Cornish’s childhood pet — a black cat, which he called “a really beautiful thing.”

“It was sitting on the kitchen table, and the sun’s coming through, and … it’s like a two-dimensional shape,” Cornish recalled. “You can only read its three-dimensionality when that two-dimensional shape moves, like a shadow puppet.

“So that was it,” he said, whispering conspiratorially about his creatures’ genesis. “It was the fucking cat.”

The beasts turned out to be the perfect complement to the Attack the Block’s slang-banging Brit dialog (which might need to be subtitled for U.S. audiences), its pumping Basement Jaxx soundtrack and its street-level action-comedy vibe. But Cornish said he wasn’t so sure the aliens would connect with moviegoers weaned on increasingly photorealistic renderings.

‘I really didn’t know whether I’d get laughed out of the house.’

“Man we were taking a bet on that,” he said, “because audiences are so used to this hyperreal CGI stuff, and I really didn’t know whether I’d get laughed out of the house.”

The aliens of Attack the Block might lack Paul’s polished CGI look — and they definitely lack his frat-boy charm — but they prove far more interesting, simply because they seem so original. Cornish’s creatures also don’t have to carry a movie like Rogen’s joker from outer space, but thankfully Attack the Block’s young actors do just fine on that front.

Here’s hoping Cornish’s movie gets picked up for distribution and you get to see his awesome aliens soon.

All sci-fi fans can name their own favorite fictional extraterrestrials. What’s yours? Let us know in the comments below.