It seems impossible to make a western nowadays without adding a measure of quirkiness to the mix, and as the title suggests, “The Sisters Brothers” is no exception to the rule. But Jacques Audiard’s film, adapted by him and Thomas Bidegain from a novel by Patrick deWitt, does better than most at maintaining an effective tonal balance, and though it can’t escape a certain episodic, picaresque tone, it’s held together by a strong cast and a propensity for finding amusing grace notes for them to play.

The plot is actually a very simple one. Brothers Eli and Charlie Sisters (John C. Reilly and Joaquin Phoenix) are gunslingers—hit-men, really—in the employ of the Commodore (Rutger Hauer), an enigmatic boss in Oregon City who employs them to kill anyone he feels has wronged him. They’re quite different people: Eli, older and more cautious, keeps close tabs on the younger, more voluble Charlie, whose drinking and whoring can invite serious trouble during their travels. They might bicker and insult one another, but depend on each other as well.

In a prologue they wipe out a whole cabin of ruffians for some unspecified offense, and then return to Oregon City for their new assignment, which involves terminating Hermann Kermit Warm (Riz Ahmed), a chemist who’d developed a formula that makes gold glisten in a stream so that it can extracted without difficulty, but then absconded with it. Warm has been found travelling to California by the Commodore’s punctilious agent John Morris (Jake Gyllenhaal), who befriends the idealistic fellow and will keep an eye on him until the brothers can catch up to them for the actual termination.

It takes the Sisters more time than anticipated, though, because of a stomach-churning accident involving a poisonous spider, and by the time they are on the trail again, Morris has decided to throw in with Warm and his high-minded notion of employing the riches he expects to accumulate to establish a model community of peace and equity in Texas. The brothers also make an eventful stop in a recently-established town called Mayfield, where they encounter the place’s strange owner (Rebecca Root). They also get to experience the opportunities afforded by big-city San Francisco, although by that time they have begun to have disagreements about what sort of future they should aspire to.

Eventually they catch up with Morris and Warm, who have set up shop at a creek they’ve staked a claim to, and after dealing with some desperados from Mayfield, throw in their lot with them, hoping to make a fortune. Things go awry, however, because of Charlie’s desire to get rich quicker than Warm’s process allows, and the result is fairly gruesome for most of the men. The brothers know that their disloyalty will bring the Commodore’s wrath down on them, and have to deal with that; but they will also find the time to visit their ma (Carol Kane).

Much of “The Sisters Brothers” has a wry, understated tone, not only because of Reilly’s performance—his introduction to a toothbrush is a goofy highlight—but as a result of the arch language the characters sometimes adopt (Charlie is infuriated by the extravagant prose Morris, a fastidious fellow, employs in his letters, but Eli calls him out on some of his own circumlocutions, too). But there are also moments of poignancy as the various characters ruminate on their often unhappy pasts and brood about what the future might bring.

At the same time, the movie does not stint on action and violence. It mostly moseys along without breaking much of a sweat, courtesy of Juliette Welfing’s lapidary editing, but there are intermittent bursts of unpleasantness that are likely to take you aback with their visceral punch—not just the gunfights the brothers get involved in (one at the start, another in Mayfield, a third at the creek, and a fourth when the Commodore’s men show up), but the brutal end to the men’s prospecting efforts and a subsequent scene involving some rather primitive surgery.

One might tire of the movie were it not for the performances. Audiard gives his actors room to breathe and fill out their characters as well as plenty of delicious dialogue, and they take advantage of it—Reilly most of all. He brings his comic chops to the part, but also his gift for understated drama, sometimes in quick succession. Phoenix doesn’t quite match him, but their scenes together have real sibling chemistry. Gyllenhaal contributes another of his enjoyably eccentric turns, though one more subdued than usual, while Ahmed keeps up with them all as the sweetest guy in the bunch. Among the supporting players, Hauer gets just a few wordless scenes, but Kane makes her small role count and Root delivers a strong, chilling turn as an untrustworthy hostess.

Audiard clearly loves the old westerns, and emulates their visual panache. The locations are well chosen, Michel Barthelemy’s production design and Milena Canonero’s costumes are scrupulously detailed, and Benoît Debie’s widescreen lensing is positively luminous—in the prospecting scene literally so.

“The Sisters Brothers” is unlikely to take a place among classic westerns—it’s more likely to be ranked alongside some of the oddball western comedy-dramas of the seventies like John Huston’s “The Life and Times of Judge Roy Bean.” But if it’s no “Little Big Man,” it’s not “The Missouri Breaks” either—it’s an agreeable mixture of tonalities that works more often than not, and you should find it worth your time travelling with these peculiar characters.