By Ed Symkus

A relaxing family vacation story morphs into a quietly riveting character study.

Sundown, directed by Michel Franco. Screening at Boston Common, Kendall Square Cinema, Coolidge Corner Theatre, and Dedham Community Theatre.

Tim Roth in Sundown.

It’s got to be considered a gamble — for producers’ coffers and for audiences’ attention spans — when a film features a lead performance that’s as understated, as expressionless, as enigmatic as the one Tim Roth gives in Sundown.

But Roth has the face and the acting chops to pull it off. The Londoner first gained notice on this side of the Atlantic three decades ago, playing Vincent van Gogh in Robert Altman’s little-seen Vincent & Theo. But it wasn’t until his first of four collaborations with Quentin Tarantino — beginning with Mr. Orange in Reservoir Dogs — that he hit his stride.

Though there’s a cast full of fine actors in Sundown, Roth is undeniably the star. He’s in practically every scene, and the camera dotes on him, whether staying up close for a look at his blank canvas of a face or pulling back to show his whole body, the measured, unhurried movements of which are nigh impossible to decipher.

Viewers will spend most of the film wondering, “Who is this guy? Why is he behaving this way?” Fortunately, those and other questions and their eventual answers are what make the film work.

It opens with an obviously well-off British family enjoying a getaway at a posh resort in Acapulco. There’s Neil (Roth), Alice (the luminous and pouty Charlotte Gainsbourg), and their two teens, Colin and Alexa (Samuel Bottomley and Albertine Kotting McMillan). It’s relaxation time. The kids order mom to stop working, to put down the phone, and she obeys, after popping some downers. Neil doesn’t have to be told to relax. He’s in slow motion, letting his flip-flops lead him around. But he also has an aloofness about him. It takes a while for the script to get around to it, but there’s eventually a revelation that he’s Alice’s brother, not her husband, and he’s come along to keep her and the kids company.

Alas, the vacation is cut short by a phone call from home. Alice and Neil’s mother has taken sick. “Start packing!” she frantically barks. Things get worse. On the cab ride to the airport, there’s another call: Their mother has died. And even worse: Neil has left his passport at the hotel.

To Alice: “You go; I’ll catch the next flight.” They do. To his next cab driver: “Take me to any hotel.” It’s a quietly shocking moment, and is when viewers will start asking themselves those questions about him. There will be more. Why does he opt for a ratty room in a bad part of town instead of the opulent resort? Why does he keep lying to his sister about his “lost” passport? Why does he eschew all family responsibilities? Why does he remain nonchalant when he witnesses a mob-style execution a few feet away from him on the beach? Why, when his now furious sister returns to Mexico to confront him, does he act as if everything is fine?

Once again, who is this guy, and why is he behaving this way? Roth’s face and body give nothing away. But his subtle acting prowess makes it clear that a lot is percolating inside his character. Late in the film, the dialogue offers an explanation to his behavior, something along the lines of how different people react to various forms of grief. A lot of what happens in Sundown is bleak and dreary. Yet, mostly due to Roth’s performance, the film remains completely absorbing.

Ed Symkus is a Boston native and Emerson College graduate. He went to Woodstock, is a fan of Harry Crews, Sax Rohmer, and John Wyndham, and has visited the Outer Hebrides, the Lofoten Islands, Anglesey, Mykonos, the Azores, Catalina, Kangaroo Island, and the Isle of Capri with his wife Lisa.

His favorite movie is And Now My Love. His least favorite is Liquid Sky. He can be seen for five seconds in The Witches of Eastwick, staring right at the camera, just like the assistant director told him not to do.