James Cameron knows how to shoot an action film like few can; and as is the case with most of his successful films, so masterfully toes the line between playful mischief and adrenaline fueled high-stakes thrills and urgency. While 'Terminator 2' probably exemplifies this dynamic best of all, the multi-genre masterpiece, 'Aliens' certainly comes close behind, and demonstrated these elements years before it.
It's relatably fun while also being intensely terrifying, and that our cast consisting of military grunts, corporate assholes, a more-human-than-human android, and strong female heroes, are all defined and constructed within the frame of the former element (think Bishop's game of "knife-scotch" or Ellen Ripley's growing mother/daughter relationship with Newt or one of the many hilarious moments involving Private Hudson), it adds that much more weight to moments that sit in the later element. The characterization is impeccable. And the action sequences, as a result, become that much more intensified, that much more captivating - the alien Xenomorphs themselves, that much more horrific in their ability to destroy the characters we've come to love (and even hate).
This movie is damn near perfect.