It isn’t the few jolts, the horror shocks that get you in “Smile.” It’s the stress. Here’s a thriller that creates unease and does a decent job of sustaining it even as it takes its sweet time in reaching the obvious resolution.

And it’s not the gruesome, self-injuring, self-skinning, blood-letting effects that make the sale. It’s the cheapest horror effect ever, every “victim” about to pass on her or his victimhood breaking into a demonic, Nicholson in “The Shining” or Malcolm McDowell in “A Clockwork Orange” grin.

Sosie Bacon of “Mare of Easttown” stars as Dr. Rose Cotter, a shrink at the Emergency Psychiatric Unit at Newark’s Mount Pleasant hospital. Emergency cases are sent her way to determine if they’re a danger to themselves or others. She may be petite, but she never hesitates to sit down, alone, in a room with people experiencing crises and exhibiting the most deranged behavior.

We hear her refer to one frequent patient as “harmless,” and we worry that she’s read him wrong, if she’s experienced enough to make that call and if she’s put herself or the staff in jeopardy in the process.

But it’s babbling, shrieking Laura (Caitlyn Stasey) who really rattles Dr. Rose and us.

When Laura says “I’m seeing things no one else can see but me,” she shouts it. When Laura declares “It’s LOOKS like people, but it’s NOT a person!” we buy it. And when she warns “It causes s–t to happen around me,” we wish Dr. Rose could appreciate the threat, not that she’d be any more able to reason her way out of it than we would, in that office with her or sitting in a seat in the safety of a cinema.

Laura proceeds to kill herself in the most gruesome fashion imaginable right in front of the stupefied psychotherapist she just met.

As you gathered from the number one movie in America’s TV commercials and trailers, when you see that demonic smile, you’re a goner. It’s a madness passing from one person to the next. “Smile” is about Rose’s search for medical, mental or supernatural reasons for this to be happening, and a way to escape the fate that’s seemingly been given her.

Writer-director Parker Finn, turning a short film into his feature filmmaking debut, gives us a thriller of austere, quiet settings, extreme closeups and pin-your-ears-back scares. And Bacon ably leads us through a health care professional’s traumatized and increasingly desperate efforts to explain the “symptoms” of what happened to Laura which are now happening to her in a way that doesn’t make her seem crazy, too.

As she’s seeing things — Laura’s smile, and others wearing that same smile — things that she’s sure are “corporeal” and not just in her head, Rose panics. As reality bends and her fiance (Jessie T. Usher), her cop ex-lover (Kyle Gallner), her irritably self-absorbed sister (Gillian Zinser) and her own psychotherapist (Robin Weigert) don’t seem to “get it,” we invest in her quest and try to puzzle out an escape clause with her.

The chilly minimalism of “Smile” limits that viewer investment, keeping events on the screen at a sort of clinical arm’s length. The casting is more”gets the job done” solid than compelling, affecting or empathy building. While Finn takes care to give his players close-ups that they dress up with gestures, tics, looks and “bits of business,” I can’t say any of the deaths here moved me.

But there’s no denying that this works as a thriller, that “Smile” is a well-crafted fright delivery system even as it slows to a crawl and stumbles into an ending we’ve seen coming for the past hour.

Rating: R for strong violent content and grisly images, and language.

Cast: Sosie Bacon, Jessie T. Usher, Kyle Gallner, Caitlyn Stasey, Robin Weigert and Kal Penn.

Credits: Scripted and directed by Parker Finn. A Paramount release.

Running time: 1:55