It’s a gimmick, a stunt, a filmmaking exercise.

Shooting your movie on an iPhone? That’s for aspiring filmmakers, somebody looking for a cheap way to shoot a movie and a headline-grabbing hook when you’re pitching it to film festivals.

Sean Baker makes “Tangerine” on the streets and seedy clubs of LA, he becomes a star director. “The Florida Project” comes next.

For a proven master filmmaker who likes to be his own cinematographer, Steven Soderbergh (he’s listed as Peter Andrews/Director of Photography in the credits) shooting “Unsane” on his iPhone had to be a fun, challenging exercise.

A paranoid (literally) thriller, he’d get the benefits of the intimacy you can achieve with a tiny camera, no bigger than your average video viewfinder on most digital cameras. He spends more time composing tight shots that get a lot of information in them, framing scenes in an arresting way that reminds the viewer, “Oh yeah, he did this with his cellphone.”

And if you watch “Unsane” on your cell phone, you’ll appreciate a picture of tight close-ups, natural lighting and action that’s right up in your face.

Does it serve the movie? Not really. The stunt is a false economy, an aesthetic “doing it one with arm tied behind your back,” a distraction. And it’s a way of reminding us of who a “star director” is or was in an era of cheap child labor age film school alumni and self-promoting hacks (Ava Duvernay and Steven S. DeKnight, this years’s Lee Daniels, Zack Snyder or Eli Roth) are studios’ preferred compliant employees.

Claire Foy, shorn of the attire, the accent and the regal makeup of “The Crown,” is the uncommonly-named Sawyer Valentini, a brittle, touchy investments analyst with a bank.

She’s new to Boston, and young — not so young that she can’t shrug off her boss’s suggestion that they attend a convention together by recognizing what he’s really pitching. Sawyer uses dating apps, and bluntly informs one bar meetup that “this evening is going to go the way you want,” but on her no-strings-attached terms.

The screenplay takes on the tint of a #TimesUp movie. And that’s before we figure out that she’s been stalked, that she moved to a new city, changed phones and uprooted her life to escape her tormentor.

And damned if she isn’t seeing his bearded, hulking presence in bars, her office, on the street. He’s gotten in her head, wrecked her sense of security and dinged her psyche pretty hard.

She reaches out to get some help, and damned if the mental health facility shrink doesn’t pull Big Mental Health Care Switch on her. Mention that yeah, maybe once you had thoughts of killing yourself, and that “boilerplate” paperwork she gives to you lets them “voluntarily self-commit” her.

Smiling, turning on the charm doesn’t work. Snapping “I have RIGHTS” to officious orderlies and nurses doesn’t work. It’s just “24 hours,” she’s assured. They have her — and her insurance — in their grasp, and bums-rushing her into a coed ward full of the genuinely disturbed where she’s threatened, bullied (Juno Temple is a belligerent psyche case who torments her) and medicated just ensures that those hours turn into seven days.

And so on.

We’re meant to wonder if Sawyer’s fears, her stalker-sightings, are all in her head. Maybe, for all the seeming shenanigans by this corporate facility, she’s in need of help.

But we’re seeing the damned stalker (Joshua Leonard) too. Her rage becomes our rage, even if her compliant reaction, almost accepting her fate even as she lashes out, fails to interest the doctors or cops in her dilemma (the stalker is now on staff there), can’t get rescued by her mother (Amy Irving, in a fierce and compact performance).

The system-wise fellow “prisoner” Nate (Jay Pharoah, cool and likable) tries to talk Sawyer through her crisis, assure her of the easiest path out. But she reacts the way any of us would. She’s enraged, humiliated. If she was bigger, she’d tear off a chair leg and try to beat her way out.

That’s an option I considered while watching her live through this nightmare. I’ve seen people use this sort of commitment to punish a wayward spouse, and it’s scary to think somebody can pull this on you.

But we don’t really fear for Sawyer, even as her stalker’s creepy fantasy unfolds and envelops her. She’s perfectly sane, we’re sure. And a smart cookie. She’ll extricate herself from this. Maybe.

“Unsane” makes a creepily watchable thriller, but it’s so light on thrillers and suspense that its Hitchcockian twist feels like an afterthought, a cheat not earned by the movie we’ve watched come before it. Listen for Soderbergh’s “Nespresso” joke. That’s for you, George Clooney.

As an exercise, it’s not as dull as an earlier Soderbergh no-budget experiment (“Full Frontal”), but it’s just as self-conscious. Film buffs will get more out of it than the casual viewer.

It’s a movie best left to Netflix, which you can stream on the same phone that the maestro filmed it on.

MPAA Rating: R for disturbing behavior, violence, language, and sex references

Cast: Claire Foy, Jay Pharoah, Juno Temple, Joshua Leonard, Amy Irving

Credits: Directed by Steven Soderergh, script by Jonathan Bernstein, James Greer . A Bleecker St. release.

Running time: 1:37