Some filmmakers rust during periods of inactivity; Lynne Ramsay arches and tenses, lying in wait like an attack dog. And attack she does, though not in all the expected ways, in her astonishing fourth feature “You Were Never Really Here,” a stark, sinewy, slashed-to-the-bone hitman thriller far more concerned with the man than the hit. Working from a pulp-fiction source that another director might have fashioned into a “Taken” knockoff, Ramsay instead strips the classically botched job at the story’s core down to its barest, bloodiest necessities, lingering far more lavishly on the unspoken emotions rippling across leading man Joaquin Phoenix’s face, and the internal lacerations of trauma and abuse they cumulatively reveal.

With the minimalism of the material providing the cleanest of canvases for the matchless technique of director and star alike, “You Were Never Really Here” isn’t the genre crossover effort Ramsay’s admirers may have feared, or possibly even have wished for. Rather, it’s a kind of art-house signal flare, reminding the industry of perhaps its greatest working filmmaker not to work often enough.

Arriving six years after “We Need to Talk About Kevin,” and emphatically consigning her unfortunate departure from the production of “Jane Got a Gun” to the recesses of memory, it’s the most contained of her features to date. Razor-cut to just 85 minutes, it’s marked by a precision that belies its allegedly hastened completion in time for its Cannes competition premiere. The absence of closing credits at the film’s first press screening truly feels like its only missing detail: Thomas Townend’s cinematography, Joe Bini’s editing and a singularly juddering, disconcerting score by Radiohead’s Jonny Greenwood all feel considered to the nth degree.

Following her marvelous “Kevin” and 2002’s “Morvern Callar,” “Here” extends Ramsay’s highly idiosyncratic style of literary adaptation, in which the written word is taken as a half-erased foundation for a more impressionistic, sensory evocation of the psyche at hand. Jonathan Ames’ 2013 novel of the same title is a crackling 90-page session of grisly shock therapy that reads very much as a ready-made screenplay treatment. Writing solo for the first time since her 1999 debut “Ratcatcher,” Ramsay turns it inside out all the same. Adding an entire closing act to its death-trail narrative, her laconic screenplay also more tenderly exposes the soul of Phoenix’s protagonist Joe: an ex-FBI agent and Gulf War veteran, now making a grim living as a contract killer specializing in the sex-slavery trade.

Introduced in the middle of one of his evidently continual suicide attempts, Joe is a grizzled grizzly of a man, wasting not one more word than necessary on clients and professional allies, and reserving what kindness hasn’t been pummeled out of him for his elderly mother (a fine, aching Judith Roberts), with whom he lives in his yellowing childhood house in Queens. Expanding significantly on the relationship as described in the novel, Ramsay picks out piquant observational details to etch a quiet mother-son bond grounded in shared anguish and victimhood, with delicate domestic nuances that are plainly the work of the woman who made “Ratcatcher.” Stray, chilling incursions of a man’s admonishing voiceover into the film’s densely layered soundtrack are all we need to fill in a backstory of severe spousal and parental abuse.

Those sharp vocal flashes are strikingly muddled with another recurring sonic motif: the steady, zoned-out counting of pre-teen girl Nina (Ekaterina Samsonov), eventually revealed as the object of Joe’s latest murderous rescue mission. The daughter of a high-ranking New York senator (Alex Manette), she has been kidnapped and put to work in the pedophile’s wing of a Manhattan brothel; Joe’s tasks are to find her, bring her home, and kill every human being who gets in the way.

It’s a simple job until, inevitably, it very much isn’t. Yet however complex the network of human and institutional corruption we subsequently stumble upon, such tangled plot mechanics are practically white noise in “You Were Never Really Here,” which is far more actively concerned with the glitchy, tortured headspace of Joe himself, as a worse-than-usual day at the office alternately gives him reasons to die and reasons to live. He may be no more Nina’s rescuer than she is his; Phoenix and the impressive Samsonov (also glimpsed at Cannes in Todd Haynes’ “Wonderstruck”) play their scenes together with the tacit mutual understanding of two children cast prematurely adrift into adult psychic pain.

Phoenix, an actor who never plays wounded the same way twice, has found a kindred cinematic spirit in Ramsay, whose camera watches, ponders and broods in perfect reflective sync with his riveting face — here clouded by a ragged hedge of charcoal-hued beard, but no less riddled than usual with fury and worry. Resisting the obvious temptation to play Joe simply as stoically hardened, Phoenix revels in taut, twitchy details of body language, while Ramsay contributes her own eccentric, sometimes hilarious inversions of the hardman archetype. This is surely the first and last revenge thriller to feature hunter and felled prey holding hands for a mumbly gallows singalong of Charlene’s “I’ve Never Been To Me.”

Ramsay’s mordant, haunted wit — quite distinct in tone from the hard-boiled humor of Ames’ novel — pops up only at selected points in what otherwise amounts to a stern, stomach-knotting stalker of a movie, one that at times fully exposes the audience to the vicious violence of Joe’s profession, and at others glides past it with the shrugging, disinterested acceptance of a long-numbed assassin. In the most brilliant of many formal coups, Joe’s one-man brothel invasion is shown from the switching, chronologically broken perspective of multiple security cameras, the background musical molasses of “Angel Baby” interrupted with every cut, as the body count silently and almost incidentally mounts.

Ramsay has made more sensually rapturous films, but this may be her most formally exacting: No shot or cut here is idle or extraneous. Townend’s calm, crisp camerawork finds rich texture and contrast in seemingly ordinary images, whether it’s a ribbon of shadow skipping across a shoulder blade as it tenses, or the velvety billowing of a garbage bag under water. Bini’s editing, seamlessly blending timelines and points of view in blink-of-an-eye strokes, gives the film the rhythm of a short fuse on a slow burn. Greenwood’s mesmerizing supporting character of a score, meanwhile, perhaps even outdoes his Paul Thomas Anderson collaborations for its instrumental range and bravado, careering from screaming strings to the discordant strum of a guitar with what sounds like a couple of snapped strings. In a Lynne Ramsay film, even the off-key elements are perfectly chosen; an exquisite, anxious study in damage, “You Were Never Really Here” knows exactly the value of its scars.