Just watched this tonight, and it definitely worked for me.
For those unaware, here's a brief spoiler-free synopsis: In a busy fast food restaurant, one phone call leads to a series of events that raise questions about why people comply with authority, and under what circumstances they'll cross certain lines. It's sort of fun going into the film knowing only that. It's based on a true story.
It's a film that tackles this one facet of human experience, and I admire that. It feels small in scope and scale, but deeply ambitious, and something about that completely works here.
It doesn't have quite the sense of dread that might have been achieved. (Maybe I'd feel different as a teenage girl.) I watched it unfold with uneasy fascination more than anything.
Everything is crafted with the sort of precision that makes things plausible, which is quite an accomplishment, because the story is completely ridiculous on paper.
In the DVD interview with the writer/director, he immediately starts talking about studies of people following orders under stress against their own better judgement. And the film does such a great job of showing how that works...
BIG SPOILERS AHEAD
For example, the fiance was in the perfect position to comply. He was just drunk enough, but more importantly, his personality seems predisposed to compliance (remember how he extra-politely called Sandra earlier about visiting his friend). He's certainly in a stressful situation, eager to do a favor for Sandra, confused about what's going on, generally vulnerable. There also must be a few dark corners of his psyche that allow him to carry out the later tasks.
The scraggly looking guy who comes in at the end is completely the opposite, in terms of personality and circumstance. He's not in a stressful situation... just stopping by to have a milkshake.
The rest of the people fell somewhere in between.
Describing it in words sounds like insufferable pop psychology, but the movie lays it all out with remarkable subtlety. I didn't expect it to be this well-crafted.