Monsters is more of mumblecore love story than a sci-fi thriller. The script was just an outline of what would happen in the scenes and the actors (a real life couple) improvised the dialogue. In this way, the film is very intimate and usually stays away from big action sequences and usually only deals with the aftermath of the chaos. For being shot opportunistically, the film doesn't stand out as guerilla. The world is carefully crafted (most likely in after effects) from the graffiti to the small cartoons telling kids to wear their gas masks.
Personally, I thought the characters we so goddamn boring and for being a real life couple had no on screen chemistry. I thought every once in a while they said interesting things like when she says