Much is made of the risk that goes into acting, of performers putting their bodies on the line, but directors are inevitably equally on the line. It’s not their bodies but their souls that their movies reveal, even if that revelation is discernible only to a smaller coterie of obsessives. Olivier Assayas’s “Personal Shopper” is a big leap and a big risk, one that puts his artistry to the test along with that of the movie’s star, Kristen Stewart; from the start, it’s an admirably bold movie that suggests an unusual and palpable urgency on Assayas’s part, which only makes its failings all the more pronounced.

“Personal Shopper,” for which Assayas won the prize for Best Director at last year’s Cannes festival, is a ghost story, an erotic thriller, and an inner journey of personal identity. It’s also a noteworthy collaboration with Stewart, her second movie with Assayas after “Clouds of Sils Maria,” in which she co-starred with Juliette Binoche. In “Personal Shopper,” Stewart is front and center throughout the film; though it has many characters, it’s the closest thing to a one-person show that the movies have offered in a while. For much of the movie—including in its best scenes—she’s acting alone, confronting a special-effects-conjured spirit, a series of menacing text messages, and a fancy wardrobe and its psychological implications. The sheer audacity of the plan is thrilling, especially following the art-snob sludgery of “Sils Maria,” “Summer Hours,” and some of Assayas’s other recent work.

Stewart plays Maureen Cartwright, a twenty-seven-year-old American woman in Paris who has taken a job she dislikes in order to pay her rent. She’s in Paris with a purpose: her twin brother, Lewis, died there several months ago, of a congenital heart disorder that she also has, and she wants to remain near the rambling villa where he lived and died, because she’s awaiting a visitation from his spirit. (The siblings had made a pact that whoever died first would give the other a sign from the beyond.) Meanwhile, her job—as a personal shopper and factotum to a wealthy celebrity-slash-model named Kyra (Nora von Waldstätten)—sends her zipping around Paris on her scooter, from designer boutique to designer boutique, dropping lots of Kyra’s money and social capital to buy or borrow the clothing and accessories in which Kyra will be seen. As for the visitations, they begin when Maureen ventures through the grand old house where her brother died—noises, cross-like inscriptions, and more—but they’re not as specific or informative as Maureen had hoped, and she plans to go back to the house and await further signs.

Maureen’s own sense of style is spare and simple—baggy sweaters, Lacoste shirts, nondescript pants, no jewelry—but she’s entrusted with the connoisseur-like selection of the highest and finest of high-fashion finery and jewelry. Her real interest appears to be art—she spends hours with a sketch pad and a dip pen, making elaborately classical architectural drawings—but she doesn’t express much in the way of ambition. She freely expresses—as on Skype, to her boyfriend, Gary (Ty Olwin), a computer-security consultant on a job in Oman—her distaste for her job, yet that job seems to be getting to her. The fancy clothing and footwear that she procures for Kyra present a constant temptation. Kyra won’t allow her to try it on, but several of the salespeople with whom Maureen works urge her to do so anyway—and, when she does, the taste of a new outer self-image disturbs and alters her inner sense of self as well.

Soon that perturbation finds a sudden and disturbing practical correlate. While preparing to take the Eurostar to London, Maureen gets a series of text messages from someone who seems to be following her, and to be making ominously erotic overtures. Her sense of fear and paranoia is matched by sexualized curiosity, and the invisible admirer’s increasingly direct pursuit fuses in Maureen’s mind with the fine clothing that she selects and tries on, leading her to consider the possibility of an encounter. Meanwhile, soon after her return to Paris, Maureen finds herself thrust in the midst of a horrific crime story that may or may not have anything to do with her own metaphysical, digital, and intimate tribulations.

“Personal Shopper” spans the axes of solitude and loss of control. Assayas stages the desperate solitude of grief, the practical solitude of the artist, the incidental solitude of the long-distance relationship, the professional solitude of the freelance solo practitioner. At the same time, he explores the uncontrollability of that very grief, Maureen’s utter dependence on signs from the dead, the tethering of an employee to the sudden orders of a capricious employer, the sense of vulnerable helplessness caused by the arrival of unexpected digital communications. The intersection of those two axes is the zero-point where total solitude meets complete lack of control: death. Maureen, living in the presence of sudden death, faces the ultimate existential adventure alone—except that, unlike others, she has Lewis, a companion who might offer some guidance and reassurance, but at the price of an even deeper relinquishment of control over her life.

Kristen Stewart is as fine an actress as she is a misunderstood one. I have always been bewildered by the old meme about her inscrutably blank expressions. In fact, her spontaneous subtlety, the passing of emotions through her with a minimal outward display, is among the qualities that put her at the forefront of her generation’s performers. Her dominant talent is being herself: her manner is spontaneous, angular, awkward; she's the geek who blossomed into a cool girl without changing at all—she was always cool and it just took the rest of the world time to catch up. She's not a conspicuously technical actor, and that's the core of her art. The closest artistic counterpart to Stewart is Greta Gerwig. What they have in common (to borrow Norman Mailer's line about Marlon Brando) is that whatever they’re about to say on their own seems likely more interesting than whatever a screenwriter has written for them. Stewart is, in effect, a mumblecore actress stuck in studio movies. Where Gerwig got started by being herself, improvising in Joe Swanberg’s "Hannah Takes the Stairs," Stewart was an international star while still a teen-ager, performing lines written for her long before filmmakers might have become interested in what she could say for herself. The implication of personal life in performance with which Gerwig launched her career might be unbearably risky for an already-established star of Stewart’s stature.

In any case, “Personal Shopper” puts Stewart through scenes and texts so closely defined as to allow her little leeway beyond the narrow margins of stage directions. As directed by Assayas, her performance abandons her powerfully emotional near-neutrality and veers and lurches between familiarly indicative, visually legible expressions. It’s impossible to know whether this is a result of too much direction or too little—whether Assayas filmed Stewart goggle-eyed with awe at her talent or deference to her celebrity and therefore left her to interpret his intentions effortfully, or whether he directed her insistently and specifically to the detriment of her artistry. In either case, the result is that Stewart’s distinctive qualities are submerged; as engaging a performer as she is to watch, she could as easily have been replaced, in “Personal Shopper,” with another of the leading lights of her generation, such as Jennifer Lawrence or Tessa Thompson. In fact, while watching the film, I found myself imagining their more overt emotionalism in lieu of the strained expressivity to which Assayas pushes Stewart.