Tools of Characterization
Characterization in Men in Black
Direct Characterization
To keep its comedic tone, Men in Black moves briskly. This speed means the film can't stop and let the audience explore the subtle nuances of the character's personalities. This film has jokes to be told, so it uses some direct characterization and just tells us who these people are.
Zed directly tells Kay that Jay has a problem with authority, but Kay counters that Jay is tough enough for the agency. As we discover, both are true. Despite increasing odds, and increasingly odd things happening, Jay continues to rise to the challenge, whether it's giving birth to a baby squid, fighting a giant cockroach, or solving the puzzle of Rosenberg's last words. His problem with authority pops up every now and then, even with Zed and Kay. When Zed suggest that Jay heard Rosenberg wrong, Jay replies, "Look, whatever, man. I know what I heard."
Likewise, Kay describes the Bug for Jay before even meeting him: "Imagine a giant cockroach with unlimited strength, a massive inferiority complex, and a real short temper." All of these qualities turn out to be true about the extraterrestrial.
Actions
When we first meet the Bug, he destroys a truck, murders a man, and then wears his victim's skin like a demented Halloween costume. Yeah, this guy's not going to be our hero, folks. His actions have told us as much.
And this goes for the other characters, too. Dr. Laurel Weaver stays up late into the night autopsying the city's dead at the morgue. We can tell she is a tad lonely because of this, not to mention the way she flirts with Jay. And even before Jay reveals the Rosenberg alien, Laurel's basically figured out the puzzle through her investigations alone: "See, I don't think this body's really a body. I think it's some kind of transport unit for something else altogether. The question is, what? Is this freaking you out?"
Finally, we see that Jay's more reckless than Kay by the way the elder agent calmly assesses each situation before acting. When Edgar Bug kidnaps Laurel, Jay leaps into action, running to catch up with the taxi. Kay calmly walks down the stairs and gets in the car. When he catches up with Jay, he explains, "He's not leaving the planet in a cab. Let's go." Kay's actions show him to be the more mindful of the two. We can tell he's been doing this for a very long time.
Type of Being
Aliens in Men in Black aren't as nuanced as they are in, say, Star Trek. In the latter, for example, Klingons are generally a militant race but peaceful ones pop up here and there. This makes it clear to the audience that Klingons are individual characters.
MiB takes a more cookie-cutter approach to its aliens. All of the aliens of a particular species are exactly the same as the others.
As we've already mentioned, Kay's able to tell Jay all about the Bug without even meeting it. Come on, Kay, what if he's part of the Bugs for Hugs movement on his planet? You don't know. Actually, he does know. Because the Edgar Bug is a bug, and Kay knows how the species as a whole operates.
In the same way, all of the worm aliens are slackers, and the Arquillians are well-intentioned. Even the ones out to destroy Earth give us fair warning. We imagine if we saw another Jake Jeebs alien that he'd be into pirating Blu-rays at the very least.
Humans are the only species that escape this kind of treatment; they're incredibly varied throughout. Laurel is intelligent, Kay's jaded and cautious, and Jay is young and reckless but kind-hearted. And Zed is, well, Rip Torn. The human characters aren't painted in broad strokes like their alien counterparts.