Writing about Jacques Audiard’s latest film, The Sisters Brothers, proves a semantic challenge. Using any variation of the verb “to be” does the work an injustice — to say that it is any one thing, or even a combination of things, fails to capture the ever-evolving life form that the film becomes. Pinning down genre and tone makes for a difficult task as well given the way Audiard allows the mood to develop organically. By all accounts, this gambit should not work. It’s a recipe for a muddled, confusing mess.

And yet, The Sisters Brothers makes for a most compelling view because of all the things that would cause most movies to fail. The shape-shifting quality makes the film a unique tribute to the Western genre. Audiard does not seek to ape, riff or revise common tropes. He seems far more concerned with honoring the spirit of the land that inspired so many others to stage thrilling tales in its vast expanses. The American West, at its core, is an embodiment of the boundlessness of the country’s spirit. The future of that territory was determined by settlers bold enough to think they could tame the terrain and forge its destiny, four of whom reside at the center of The Sisters Brothers . Those with the gumption to reshape the world also needed to believe in their own capacity for self-reinvention. By allowing the film’s form to mimic the constantly remolded clay that is the American frontier, Audiard crafts the ultimate tribute to it. (Ironically, he shot the film across Europe.)

Perhaps it’s no surprise that the other guiding creative force behind The Sisters Brothers was John C. Reilly, whose own career serves as a testament to the power of versatility and flexibility. His character Eli Sisters encapsulates the unique blend of humor and sadness that Reilly brings to the table in films as varied as Magnolia, Chicago and Step Brothers. Reilly’s characters often possess a pathological need for acceptance, and they are willing to offer extreme loyalty and devotion in order to receive it from others around them. This devotion usually starts off with a comedic bent, or — at the very least — Reilly plays these notes in a more pathetic register. In The Sisters Brothers, like many of his more dramatic turns, the actor converts that humor into tragedy – providing a walloping twist for an audience.

As one half of the titular Sisters Brothers, Eli serves as the more stable portion of the duo. Though Eli is the elder statesman of the two, he takes a backseat to Joaquin Phoenix’s Charlie, a loose cannon and incorrigible fireball. (Fans of the actor’s controlled chaos as Inherent Vice’s Doc Sportello will find much to love here.) Their partnership as debt collectors for a nebulous figure known as The Commodore has developed a familiar pattern. Charlie acts impetuously, and Eli has to clean up the mess left behind. The behavior carries over from their personal lives, too, where Charlie severed family ties and Eli followed his lead.

Over the course of The Sisters Brothers, Eli has many an opportunity to get introspective, a rarity in the regularly psychologically-averse Western genre. He broods openly about their fraternal bond and expresses a yearning to go honest. Yet when Eli posits the idea of opening a store together with Charlie, his brother laughs him off. Charlie lives on the thrill of the hitman’s lifestyle; in the film’s opening scene, he asks Eli how many people he thinks they killed as they walk away from a scene of their slaughter.

Charlie and Eli map well onto the Western archetypes of the cowboy and the sidekick, respectively, but Audiard hardly seems concerned with using them to comment on the mythic figures. The brothers are not necessarily anachronistic personalities, but their speech patterns resemble contemporary vernacular more than period-specific dialogue. This has the effect of separating them from the genre, making the Sisters feel like they wandered into the Western genre rather than arose from within it. Their slight alienation might feel weird at first, but the deliberate maneuver pays dividends when the sands shift in the second half of The Sisters Brothers.

The film starts off with a seemingly straightforward genre set up. Eli and Charlie must track down the huckster chemist Herman Warns (Riz Ahmed) to collect his debts to the Commodore, a task with which they receive assistance from British prospector John Morris (Jake Gyllenhaal). Warns makes for a wily adversary, and he astutely makes more of Morris tracking him. Rather than quarrel with his pursuer, he lets him in on the master plan. Warns has devised a chemical compound to remove labor and luck from the equation when searching for gold in the California streams, the profits from which will fund a new utopian community in Dallas. By the time the Sisters brothers finally catch up, Warns and Morris are partners in their own right now that the latter has given himself over to the promise of reinvention. (Nightcrawler buffs will relish the role reversal in the dynamic of persuasion.) This deescalated hunt changes the field of play, erasing rivalries and uniting them around the promise of abundance.

The get-rich-quick scheme comes with a cost, as they always do: Warns’ chemical causes an intense burn when making skin contact. The back half of The Sisters Brothers slows down a bit to examine not only what the men do but also what it does to them. Audiard finds the real drama of the film in how the seemingly boundless promise of the land collides with the very real limitations of the human imagination and body. Were he simply dissecting or deconstructing the Western, Audiard’s film might relegate these devastating insights to a navel-gazing conversation of genre conventions. But because he opens it outwards, The Sisters Brothers can be more. It is more, as a matter of fact.

Follow Marshall Shaffer on Twitter (@media_marshall).