After Lex catches Miller exploring the abandoned whaling station on his own, she can't resist teasing him about being startled by a penguin.
Lex: Careful. They bite.
When the team finds the Predators' shoulder cannons. Stafford isn't very happy that the experts in archeology brought on the mission specifically to identify any strange or unusual artifacts can't identify the cannons.
Miller: Well, yeah, it is a good thing, cos' this is like finding Moses' DVD collection.
While moving through the pyramid looking for Scar (the surviving Predator), Alexa jokingly asks Sebastian "How do you say 'scared shitless' in Italian?" He answers with "Non vedo l'ora di uscire da questa piramide con te, perché mi sto cagando addosso... More or less."
Translated, that means "I can't wait to get out of this pyramid with you, because I'm shitting myself."
From the Alien vs. Predator 2 video game, the early Marine missions are filled with tension. You're exploring an abandoned colony complex, not unlike the one from Aliens, with signs everywhere that Xenomorphs got loose and rampagey. You're separated from the rest of your squad, wandering through the dark, alone, but can still hear them on your comms (though you can't respond.) Acid burns and damage are everywhere, indicating one hell of a fight. Then, you hear over your radio:
Random Marine: No way, somebody tell me that's just a boot, that better be a boot!
A deleted scene from the first movie is actually called "Predator Humor". When Scar is taking apart the xenomorph to make Lex a weapon, he gestures for her to come close. Slowly he drags his knife down the Alien's exposed brain, then jabs an area, making the alien's mouth-tongue shoot out, startling Lex. Scar doesn't laugh out loud, but his body language is enough to tell that he's very pleased with himself.