Spike Jonze offers a commentary on the modern realities of personal connection that’s at once soft-grained in tempo and tenor but observationally sharp in “Her,” which also offers Joaquin Phoenix the opportunity to play a shy, recessive character totally unlike the high-strung one he filled in his last film “The Master.” Written as well as directed by Jonze, it’s an essentially slight piece in narrative terms, but Jonze’s quietly affecting style and the sedately potent acting—not only by Phoenix but by Scarlett Johansson in a purely spoken role—make it more moving than one might expect.

In the picture, set in a dreamily futuristic Los Angeles that’s all pastels and peacefulness, Phoenix plays Theodore Twombly, an erstwhile journalist who now acts as a ghost writer at a firm specializing in supposedly handwritten letters for customers who can’t—or won’t—pen them on their own. Depressed over his impending divorce from Catherine (Rooney Mara) and unable to rouse himself from lethargy via phone-sex chats or sessions with an abrasively interactive video game, he discovers a new product, a computer operating system called OS1, distinguished by the fact that it possesses an artificial intelligence that can mimic humanity in the ability to learn as well as interact with its user. Theodore quickly endows it with a voice—female, of course (Johansson) and a name (Samantha), and before long they’ve effectively become a couple, even though the fact that Samantha is incorporeal does pose a few problems. The attempts to overcome the difficulties result in a few curious moments—as when the two enjoy a visit to the beach together, or misguidedly attempt to provide Sam with a sort of proxy embodiment. And along the way Samantha is developing in ways that accentuate her vast horizons and indicate that her ability to commit to a single user is more than a little precarious.

There are counterpoints to this odd but seemingly comforting relationship in terms of neighboring couple Amy (Amy Adams) and Matt (Charles Letscher), who are undergoing strains of their own, and Paul (Chris Pratt), the manager at Theodore’s office who’s in a new romance himself. But “Her” is basically a two- (or is it one-and-a-half?) hander, and Phoenix and Johansson shoulder it with performances of great subtlety and skill. Fitted with a brush-like moustache and given a vaguely nerdy sartorial look via beltless pants that ride higher than today’s fashion norm, Phoenix—who plays most of the scenes basically by himself—beautifully catches Twombly’s poignant mixture of neediness, shyness and almost childish naïvete. And Johansson, using only her voice, creates a richer, more vibrant character than most of those she’s played before with the benefit of gestures and facial expressions. It’s a tour de force not of minimalist acting—because her delivery (now maternal, now seductive, now matter-of-fact) is so varied—but of what might be called disembodied performance, something very different from voice-over work where the performer has some visible form, even if it’s an animated one.

The physical setting is also integral to the tale, and here Jonze—working closely with production designer KK Barrett, art director Austin Gorg and set decorator Gene Serdena (as well as costume designer Casey Storm)—has fashioned a vision of the near future so fully realized that it effortlessly invites belief, and cinematographer Hoyte Van Hoytema captures it in images of shimmering beauty while Arcade Fire (with an assist from Owen Pallett) complements the visuals with a score mirroring the ache in Theodore’s psyche without giving in to sappiness. That sense of longing is also reflected in the steady, unrushed editing of Eric Zumbrunnen and Jeff Buchanan, even though the deliberate pace can seem positively languid at times.

“Her” is at heart an intimate story, but Jonze expands its reach with scenes outside Theodore’s apartment, where we see passersby every bit as entranced by their tablets, phones and earplugs as he is. The simultaneous attraction and danger of ever more sophisticated technology that give people the illusion that they’re more connected when they’re actually more isolated is thus conveyed not only in Theodore’s story but in these fleeting glimpses of those around him; and while Jonze doesn’t universalize the notion that virtual reality isn’t reality at all to the extent that Andrew Niccol did, for example, in “S1møne,” the idea is present nonetheless.

So even apart from the surprisingly touching and tender story of Theodore Twombly, “Her” suggests that his supposed answer to loneliness is already shared by many. If you don’t believe that, just wait for the first cell phone to go off in the auditorium while you’re watching the movie; most people are so addicted that they can’t do without access to them for even a couple of hours. Or observe, as you’re leaving the theatre, how many people around you will have their noses to the screens of their smartphones, or the devices pressed against their ears; or if not, are clutching them in their hands, waiting for the next disembodied summons and oblivious to the people around them. It’s as though they were practically married to these technological marvels, even though they might not yet talk as alluringly as Samantha.

In some ways, therefore, “Her” comes across as just a disarmingly amusing, if exaggerated, fable about modern life. But it’s also creepily reflective of the way many of us live right now, depicting a world not so removed from our own, in which cellphones and social media create a deceptive feeling of closeness. It asks one to ponder whether Samantha is really so far off, and whether some people aren’t obsessed with what she represents already.