The Stanford Prison Experiment, which took place in 1971, remains one of the most iconic 20th century studies of human nature. In it, a group of Stanford students were split into two groups — half of them guards, half prisoners — and positioned in a section of the university outfitted as a prison, where their interactions were monitored using closed-circuit cameras by a group of Stanford researchers led by psychology professor Philip Zimbardo.

The experiment was meant to last two weeks; it was shut down after just six days as the study’s de facto guards began enacting tactics of psychological torture on the prisoners. The study — and its results and findings — is still taught in many introductory psychology psychology courses.

Because the experiment itself is such a compelling and well-documented piece of academic history, the task in front of director Kyle Patrick Alvarez was not so much putting together a linear narrative of it, but rather, capturing the sense that it was (and remains) at once instructive, intelligent, and disturbing. He succeeds, and it’s mostly on the strength of his cast.

In the film, Billy Crudup plays Zimbardo, the still-revered psychologist who actually — and unwittingly — allowed himself to succumb to his experiment: Crudup-as-Zimbardo gives no obvious sign that he has become part of the study, meaning the lines between reality and experiment for his subjects continues to blur. The prison guards assume character almost immediately — in one case, quite literally: guard Christopher Archer, played with terrifying venom by actor Michael Angarano, turns himself into a no-punches-pulled, John Wayne-esque drill sergeant as soon as he dons his guard’s uniform — as the prisoners are shouted, intimidated and manipulated into submission, and into fear.

It’s fascinating to watch, but also horrifying; two of the Stanford experiment’s participants quit early due to psychological distress — actor Ezra Miller, as one of these subjects, has an occasionally unwatchable mental breakdown over about 30 minutes of screen-time. The prison guards, despite a few of their apparent predisposition for authoritarianism, are never a united front, and Alvarez chooses to convey this not through confrontation or dialogue, but extended, jarring closeups on the faces as they witness the abuses of others.

For a film whose central tenet is the intricacies of human nature to truly work, it required expert casting. This, it has: the young men who comprise the film’s chorus of guards and prisoners aren’t exactly a bunch of unknowns — you will have seen Miller, for instance, in Trainwreck, while Angarano is part of the cast of The Knick — but they’re not an ensemble of easy-to-recognize A-listers. The skill each of these young men enacts in succumbing to his role makes The Stanford Prison Experiment an engrossing experiment in and of itself.

Indeed, Alvarez’ film would almost qualify as a psychological thriller if it weren’t a faithful, slightly dramatized reenactment of history, and if we didn’t already know how it ended. Ultimately, the film — and the experiment itself — comes to one salient conclusion, which Alvarez emphasizes in the film’s final scene showing Miller grilling Angarano on his abuses: the things we don’t know about ourselves are often the most ugly.