“Daddio” is a cinematic seminar in the value of movie stars.

A variation of the “Night on Earth/Taxicab Confessions” formula, it puts Sean Penn behind the wheel and lets Dakota Johnson hold her own with him from the back seat.

Its sole power to dazzle is in the things a cab passenger will tolerate in terms of frank, coarse conversation about sex and the city. In a drawn-out, melodramatic chat, our rider lets her philosopher, confessor and psychoanalyst cabbie “read” her, flirt and offer unsolicited advice on his “last fare” of the night drive from the airport into Manhattan.

By turns creepy, sexy and forlorn, the picture is made mesmirizing by an Oscar winner doing his best world weary and edgy act for one of the screen’s great beauties, with her showcased in a performance of reactions, and counters, framed in adoring close-ups.

Christy Hall, a TV writer (“I Am Not OK with This”) who scored the assignment to adapt the novel “It Ends with Us” for Blake Lively and Justin Baldoni, makes her writing-directing debut with this simple, chatty two-hander.

Her way with a pithy turn of phrase — “bucket list” wishes, philosophical observation and crude-enough-to-be-a-come-on sexualized conversation — must have made this an easy sell.

Our rider is from the city. “Your little outfit gave it away,” he says, after thanking her for not staring at her phone in a long, accident-delayed “fixed rate” ride. “You can handle yourself.” She must be “a New Yorker who pays attention.”

But she is on her phone, getting sexted by her paramour. There’s resignation in her eyes at her lover’s over-eagerness. She’s just flown in from “back home” in Oklahoma and her sexter is in the mood.

As the cabbie’s questions and observations grow more and more personal, maybe we wonder if she should be passing on the name off the hack license displayed in the back seat. More than a few remarks give the driver a stalker vibe.

“Looking like a family man is more important than being one,” he growls. Cabs and cabbies “are like f—ing BLOCKbuster,” he grumbles. The self-driving taxi/app in on the horizon, and that’s the end of his profession.

He’s got a hint of bitterness, but he makes a lot of eye contact. She stares off into the night, smiles, brings her vocabulary down to his street argot and lets on about her work, her life and whoever it is who keeps begging her for texted nudes in the middle of a cab ride.

The conversation can be playful — “I can’t be a knowitall if I don’t know nothing.” — and insightful, mostly from his cynical, man-of-the-sexual world end.

“Don’t ever say the word ‘love.'”

“I’m not THAT girl!”

The script’s simplicity is both its beauty and its trap. Two players, lots of two-shots and soulful, reflective close-ups and twists that are hardly surprising are bound to cause a little impatience. The intimate setting is more myopic than claustrophobic. The stakes seem low, and are.

But in a world where people still take cabs and not Lyfts, where sexting is still a thing and guarded, hardened New Yorkers don’t just make eye contact, but talk about their pasts, their fears and desires, “Daddio” works.

And Penn and Johnson, confined to a single setting, let their star power do the heavy lifting and create possibilities out of nothing but their screen appeal, their magnetism and their ability to become characters just far enough removed from their off-screen personas to be interesting.

Rating: R, nudity, profanity, adult conversation

Cast: Dakota Johnson, Sean Penn

Credits: Scripted and directed by Christy Hall. A Sony Pictures Classics release.

Running time: 1:40