Acting, it’s said, is all in the reacting. When a performer has truly and completely inhabited the headspace of the character he or she is assaying, reactions logical with the outline of the character become second nature. The moments in which a character simply listens and responds to the events transpiring around them always reveal which actors and actresses know what they’re doing, and which are doing their best.

In Denis Villeneuve’s taut new drug-war thriller Sicario, Emily Blunt perfectly illustrates this principle. She spends long stretches in total silence, and yet the complex interplay of emotions flitting across her face communicates more than an inch-thick stack of script pages ever could. In a hyper-tense showdown setpipece about halfway through the film, Blunt’s character — straight-shooting FBI agent Kate Macer — hangs in the backseat of her squad’s cruiser while they dispatch a carful of heavily armed men on the highway at the Mexico-U.S. border. The reasons why she’s in the car instead of on the street with the others matter, and we’ll get to those, but what she does in the car ends up being no less vital. She peers out the window to get an unobstructed view of the shootout, and when the terrible crack of the first shot sounds out, Blunt flinches. It’s such a minuscule move, only lasting a fraction of a second. But in that one jolting flash of genuine fear, Blunt’s character overturns the narrative tradition of bad-assdom.

Sicario is no typical tale of cops and criminals. The war on drugs# makes great compromisers of all ensnared within its web of moral inexactitudes. Kate gets strongarmed into joining a decidedly shady inter-agency task force dedicated to disrupting the operations of a major Mexican cartel, left with no real other choice.

The two operatives leading the squad keep Kate in the dark as long as possible, coughing up an affiliation with the Department of Defense to placate Kate’s repeated requests to know their deal. This is a lie, one of many that Matt (Josh Brolin, both genial and creepy) and Alejandro (Benicio del Toro, ice water in his veins keeping him cool under the southwest sun) tell Kate to prevent her from being an effective enforcer of the law. Blunt’s Kate is the polar opposite of a cop on the edge who doesn’t play by the rules. She’s a cop who wants nothing more than to back away from the edge and play by the rules. She’s not a bad-ass, she’s a good-ass. She’s not a loose cannon, she’s a… tight cannon? You get the picture.

Diverging from the scores of tough-as-nails female characters that have preceded her, Kate responds to the scandalizing violence erupting in every direction not by kicking ass and/or taking names, but by panicking and adhering to protocol.

From the outset, Kate senses something unsavory about the circumstances into which she’s been thrust. She’s sent on a mission to El Paso, and as the sign welcoming her to Juárez, Mexico sharpens in her vision, she can feel things getting less and less legal. In the past, police thrillers have tended to prize values of decisiveness, strength, and independence in their heroes.# Kate’s certainly not unskilled with a firearm, but she’s uncomfortable. She’s human, which is to say she’s breakable; while she assumes a steely front for her colleagues’ sake, in private interludes, she’s petrified that she’s become inextricably tied to something larger and more fearsome than she. When Kate begins to realize the seriousness and enormity of her squad’s black-ops activities# she does the least-chill thing of all and tattles to her immediate superior. In the ultimate denial of her individual agency as a character, that overarching metric which badassery is measured, her higher-ups tell her to pipe down and follow orders.

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Blunt’s not entirely unfamiliar with this particular narrative subversion, either. In 2014’s covertly brilliant Edge of Tomorrow, she played Sgt. Rita Vrataski, a reputedly unkillable supersoldier alternately known as both the Angel of Verdun and Full Metal Bitch, nicknames suffused with reverence for extraordinary power, and stone-cold motherfucker tendencies. Her legend turns out to be a fine fiction; the woman’s literally unkillable, granted the gift of eternal do-overs and using her anticipatory knowledge of battle outcomes to emerge victorious. In actuality, she’s a reformed coward who’s benefitting from holding all the cards. Her non-badass actively questions the myths that help build such figures into larger-than-life titans that deign to stride amongst mortals.

In Sicario, the roiling moral turpitude of its heroine and the relentless intensity of the drug war that threatens to swallow her whole freely switch between main-attraction and side-show status. If audiences fixate on Blunt’s ethical garden path to hell, the unforgiving horrors of the drug war sneak up on them. The audiences that came for a gritty action picture will get a slightly-off vibe from the film, and only then realize that theirs is a reluctant protagonist.

It’s a tricky beast, and it’s Blunt’s layered performance that bolsters the film’s brilliance; in the hands of a lesser actress, the script would present as poorly-structured or inconsistent. But her skill as Kate transcends scripting, good or bad. The crisp alertness of true-blue terror in her eyes — there’s no writing something as real as that.