There’s more than one way to be a passenger in life, not all of which involve vehicles. That’s certainly the case when it comes to the protagonist in Carter Smith’s propulsive new thriller, which functions like a therapy session on wheels between a murderous shrink and an unwilling patient. If the two of them don’t exactly paint the town red during their would-be getaway, it’s only because their journey was precipitated by a shocking act of violence in their place of business and they now have to keep a low profile. “The Passenger” does as well, and proves sneakily effective with its low-key approach to several kinds of violence.

Before anything overt occurs, the film unsettles us via the color scheme of the grimy fast-food joint where Bradley (Johnny Berchtold) works: a mix of unappetizing yellow, orange, and brown that looks to have been designed at the height of the ‘70s and never updated in the decades since. He’s a lock for employee of the month, not that that’s saying much: Benson (Kyle Gallner of last year’s surprise hit “Smile”) mostly keeps to himself, while Chris (Matthew Laureano) is a bully more interested in coworker Jess (Jordan Sherley) than he is in collecting a paycheck. To call it a toxic work environment would be an understatement, especially after a triple murder results in Bradley hitting the road as Benson’s passenger/hostage. Suffice to say that his promotion to manager at another location is being put on the back burner.

Gallner does exceptional work as the unstable force who sets these events in motion, particularly in his first scene behind the wheel with Bradley in the passenger seat. Doing a bit of mental math, he thinks aloud while calculating how many hours it will be before anybody (whether a customer, fellow employee, or police officer) realizes what’s in the restaurant’s walk-in freezer and who is most likely responsible. “There’s something about you,” Benson tells his captive. “There’s something fixable — and I believe that.” That dynamic is ultimately the film’s center: Benson spares Bradley because he considers him intelligent, but spends much of their time together castigating him for also being a pushover who refuses to stand up for himself.

As the other half of the dyad, Berchtold complements Gallner’s vibe with a nervous energy that radiates through every mumbled sentence. “The Passenger” is primarily a two-hander, but an impressive Liza Weil, as a figure from Bradley’s past whom Benson forces him to confront on their day-long journey, makes the most of her modest screentime — it’s difficult not to wish her role had been expanded. But less really is more sometimes, a philosophy that Smith (who segued from the studio horror “The Ruins” to indies “Jamie Marks is Dead” and “Swallowed”) and screenwriter Jack Stanley put to skillful practice: The film is effective not despite how linear and bare-bones it is, but because of that.

“The Passenger” occasionally eases the tension with humor, as in a recurring bit involving characters asking Bradley his last name, only to realize it’s actually Bradley: His first name is actually Randy, and he was too timid to point out the error when his name tag was made. That’s our mild-mannered hero in a nutshell, and the most uncomfortable aspect of his relationship with Benson, who, despite committing unforgivable acts, isn’t entirely wrong about Bradley.

That all of this stems from a childhood trauma glimpsed in an opening flashback might be clichéd, but Smith and Stanley satisfyingly unravel that particular narrative knot with help from Weil’s moving performance. Like almost everything else in the film, it simply works. This is the final title in an eight-picture deal between Blumhouse and MGM+ that has also included the likes of “A House on the Bayou,” “Unhuman” and “There’s Something Wrong with the Children.” The partnership has mostly flown under the radar, but if “The Passenger” is any indication, it probably deserves a second look — just as this movie deserves a proper theatrical release.