Sean Baker didn't want a star. The director and co-writer of 2015's indie hit Tangerine was struggling to find the right actress for his follow-up, The Florida Project, about the hard-luck denizens living in an Orlando-outskirts motel. Thanks to Tangerine's critical success, Baker could easily have landed an established performer to play Halley, the impoverished but timber-tough single mom whose financial desperation leads to some calamitous decisions. Yet, none of the contenders felt right.

"We were discussing all of the 20- to 24-year-old hot names in Hollywood, and even some pop stars," says the 46-year-old Baker. "But I was concerned that the audience was going to be taken out of it every time they saw a recognizable face in this role, because of her struggles. And I love bringing fresh faces to the screen." The casting call he needed, it turned out, would come from his smartphone.

One day, while browsing Instagram, Baker inadvertently landed on the feed of Bria Vinaite, a New York-based clothing designer and pot-lover whose posts channeled Halley's hard-won carefree spirit. "There was a video of her jumping around the backyard, which made me laugh," Baker remembers. "And there were a lot of [clips] where she would talk directly to the camera while smoking a blunt." (Other highlights of Vinaite's pre-Florida feed: An appropriately blunt list of the ways to make her happy, and a lip-synced performance of Trillville's 2004 hit "Some Cut"). "She didn't have a care, and didn't have a filter," Baker notes.

She also didn't have an any acting experience—though that fact that didn't prevent Baker, who'd previously employed Vine and Soundcloud to find new film talent, from reaching out to her via DM. "In the past, an actor would have to move to LA, get a manager and an agent, and get in that database," Baker says. "But now, you can see people from all over the world, if you take the time." At the time he reached out, Vinaite was living in Miami, using Instagram to keep in touch with friends. "I was really bored, so I used to post silly videos just to pass time and make myself laugh," the 24-year-old recalls in an email.

When she got Baker's message, she says, "My initial reaction was disbelief! It just seemed too good to be true, and so surreal. I don't watch a lot of films or TV, so I had not heard of his films. But the day he reached out, I watched all of them the same day"—including Tangerine, a brisk, affecting, late-night L.A. travelogue revolving around a pair of transgender sex workers.

Vinaite eventually agreed to fly to Orlando to meet with Baker, who shortly afterward offered her the role. Before filming began, though, she underwent nearly a month of rehearsals, where she worked alongside the movie's other breakout star, 7-year-old Brooklynn Prince, who stars as her cautiously big-hearted, delightfully big-mouthed daughter; both actors share numerous scenes with Willem Dafoe, playing the hotel's kind yet put-upon caretaker.

While there's an unforced naturalism in all three performances, Vinaite's role may have been the most challenging. Halley is both charmer and tempest—opening her arms to the world in one moment, raising her middle fingers to it in the next—yet even when she's at her worst, she remains recognizably vulnerable. The confidence and casual brazenness that Baker first spotted in Vinaite's Instagram has transferred, lovingly and thoroughly, to the big screen. "Bria didn't want to fail at this," says Baker, who cast several other non-professionals in the film. "She knew she was green."

Vinaite and her castmates are among the many stumbled-upon joys of The Florida Project, which was filmed at a purple-hued Kissimmee motel, where Baker and his crew were confronted by all sorts of real-world surprises, from sky-framing rainbows to tourist-trapping helicopters. (The choppers seem to take off every 20 minutes in The Florida Project, as if to noisily remind the motel's struggling residents that a happier existence is out there somewhere). Baker, who famously shot Tangerine on an iPhone 5, captures all of this on 35-mm film, surrounding his characters with sun-popped colors and gorgeous highway vistas.

It's the kind of visually, emotionally panoramic story that's best absorbed on the big screen, and it's anchored in no small part by Vinaite, whose unlikely turn has charmed not only movie critics but also Drake, who hung out with Vinaite at last month's Toronto Film Festival (a moment captured, of course, on Vinaite's Instagram feed). "Now she has the acting bug, and she has a manager," Baker says. "It's going to be an interesting thing to watch."

More Movie Magic

• Angela Watercutter talks to Angela
  Robinson, director of the new film Professor Marston and the Wonder Women.
• Brian Raftery profiles a film
  obsessivewho creates ridiculous statistical analyses of action movies.
• Adam Rogers explores the chaos that Rotten Tomatoes has
  wreakedon Hollywood's creative engine.