Truth is a necessarily elusive notion in espionage and, when it comes to actionable intelligence, an endlessly, sometimes dangerously transitive commodity. Defining “actionable intelligence” is itself a slippery slope, for that which prompts action could simply be a belief espoused by the room’s loudest, or most persuasive, voice.

Even then, tough questions. How does one reconcile a lead driven more by hope than knowledge, with an almost incalculable risk of inaction? And where might that take you? Nowhere? Into a fatal trap? Or, as we find at the exhaustive end of Zero Dark Thirty, director Kathryn Bigelow and screenwriter Mark Boal’s meticulously engrossing and engaging film, to an empty uncertainty that follows the death of the quarry?

This creative duo’s follow-up to The Hurt Locker is simultaneously a long-form journalism piece, docudrama, thriller, action film and character study. And the fate of the prey is hardly a spoiler when his name is Osama bin Laden, whose pursuit over a decade-long, post-9/11 manhunt is rigorously dissected here.

Zero is procedural enough to be broken into chapters, but it’s never a dry, dispassionate presentation of exhibits and evidence. And although the film concludes with the bracing, muscle-knotting intensity that’s Bigelow’s stock-in-trade, it’s weary and wise enough to avoid the obvious flag-waving while depicting the Pakistan raid in which bin Laden was shot and killed. No, this film’s unshakably monolithic power and suspense derives not from the bullets fired but the dominoes felled to arrive at that moment.

Horrifying but not exploitative, its prologue is a reminder of the high-running emotions of Sept. 11, 2001 — scanner traffic, phone calls, voicemails in the aftermath of the attacks. As a nation under siege, this was our chatter, our intel. Its mounting pitch and hum is a chilling contextualization, mirroring the thrum of noise through which our fictionalized protagonists spend the next 10 years sifting.

That’s a far cry from Zero’s original incarnation, which was to have covered just the three months immediately following that morning. Boal and Bigelow had prepped to chronicle the December 2001 Battle of Tora Bora — a five-day military firefight in the mountains of Afghanistan, from which bin Laden escaped to Pakistan.

But when word of the al-Qaeda founder’s May 2011 death dominated the news cycle, Boal and Bigelow regrouped on the fly. Boal, a veteran journalist, tapped many of his same CIA sources for the details of how the intelligence community eventually led SEAL Team Six to the sand-caked door of a Pakistan compound.

Not surprisingly, Zero now has immediacy some have pegged as insurgency. Just last week, the Senate Intelligence Committee announced an investigation into whether Boal and Bigelow received inappropriate access to classified information. And before those outside Hollywood’s inner circle saw the film, debate raged in Washington’s beltway as to its endorsement of “enhanced interrogation” — bureaucratese for “torture” — as the gateway to bin Laden’s whereabouts.

“Everyone breaks, bro. It’s biology.” Such is the deceptively dulcet tone of Dan (a mesmerizing Jason Clarke), a CIA interrogator at an off-book “black” site in Pakistan. It’s there that the audience, along with newly arrived CIA agent Maya (Jessica Chastain), is disturbingly oriented into his day-to-day work.

Although limited to the first act, Bigelow’s depiction of waterboarding, sensory abuse, stress positioning and humiliating nudity is astoundingly difficult to watch — as it should be. Does Dan’s prisoner eventually divulge the alleged whereabouts of bin Laden’s personal courier — critical intel onto which Maya latches over her long pursuit? Yes, but it’s in a moment of mental cunning disguised as relative kindness.

And given an eventual choice Dan makes, his ominous mantra might as well apply to the men and women who employ such methods. The movie neither condones nor condemns his actions. It simply presents the circumstances and allows viewers to arrive at unguided judgment — less about torture’s efficacy in eliciting information about bin Laden than about whether it was worth the moral quagmire.

Maya, too, is unnerved by what she sees at the black site, but she knows to leave the room or even hint at discomfort would be a sign of weakness. That’s a perception she will never feed, for she’s very much aware she is “the other” — which is to say a woman in a field dominated by testosterone and machismo.

This builds to neither a showboating, crowd-pleasing rallying cry for feminism nor an ego-feeding, stand-in metaphor for Bigelow as the rare female action filmmaker. Instead, it’s just a quietly motivational character trait, subtly underscored in Boal’s script and Bigelow’s staging — the way Maya is shoved into the corner of a conference room at the most pivotal point of her investigation or how dismissively someone identifies her in the moment of accomplishment for which she’s yearned.

It’s but one nuance of Zero that draws the most magnetic performance yet out of Chastain, a chameleonic character actress. It’s work that often simmers rather than erupts, all the trickier a balancing act for it, and Chastain remains resolutely compelling throughout. Maya’s lone moment of exhalation at film’s end is born not of triumph but psychological unmooring — an unforgettable depiction of all, and just how much, her passage beyond dedication, and into obsession, has cost her.

Zero’s detractors might peg Dan and Maya as detached ciphers — icy and impenetrable pawns whom Boal and Bigelow strategically move for the sake of their narrative endgame. A counterargument: When eagerness is the deadliest emotional investment someone can make, isolative tendencies become a shield.

As Maya and Dan seethe, so does the film. Catharsis is infrequent. Victories are few. The clash between statistics and instincts yields casualties. And heart, as often defined by Hollywood, here becomes just a physiological midpoint between brain and gut — trusted at the risk of peril. There is a third-act moment in particular that, in more conventional hands, would have led to an Oscar clip for both Chastain and Clarke. It merely speaks, in a whisper, to realignments of loyalty and certainty before moving on to the next internal struggle.

These are characters often self-ensnared and entrapped by their confidence, let alone by the street-level terrorists who sometimes prey upon it and threaten their lives. In some ways, this rough beast is the anti-Argo, an exceedingly entertaining film that cut its tension with satire and melodrama. Here, the identifiably human elements are simply rubbed so raw that it takes some work to identify them.

And although the Navy SEALs aren’t introduced until the film’s final act, their elemental personalities come into play in unexpectedly affecting ways — all courtesy of an actor best known as the lovable goofball on network TV’s best sitcom.

It has been a banner year for supporting players from “Parks & Recreation” shedding their slapstick skin — Nick Offerman in Smashed, Rashida Jones in Celeste & Jesse Forever, Aubrey Plaza in Safety Not Guaranteed. But Chris Pratt emerges in the most surprising way — perfectly underplaying moments of skepticism without dismissiveness, grief without sadness.

He and his team have shed crimson chasing this particular white whale before, and he simply, shrewdly works to hear what will convince him that Maya’s intel is something different. As for their raid, it’s an enthralling episode in which the precision of a surgical strike teeters on the verge of act-of-war chaos and violence.

Our journey into a heart of darkness to demand justice for thousands of lives may have been necessary. But Zero Dark Thirty understands it will never truly be done and still carries a weight and burden long beyond the moment of one man’s death. The irreducible truth about it is certain: It’s one of the greatest movies of 2012.