Lynn Shelton's Humpday is no stranger to The Stranger, and vice versa. Last September, Shelton became the sixth recipient of The Stranger's annual Genius Award for Film, an honor that earned the Seattle-bred filmmaker a big cake, a big party, and a $5,000 cash prize. At the time of the 2008 Genius Awards, Shelton was wrapping up filming on her third feature, Humpday, which takes its name and central plot device from HUMP!, The Stranger's annual amateur-porn competition, wherein adult films made by locals are screened for packed houses who rank their favorites. After the final screening, the master tapes are destroyed before the audience's eyes. (As HUMP! host Dan Savage puts it, "Be a porn star for a weekend, not for the rest of your life on the internet.")

But these facts—more points of interest than conflicts of interest—are the least interesting thing about Humpday, a tiny, local independent film that's met with international success, from a splashy premiere at Sundance (where the film earned raves and sparked a bidding war among distributors) to inclusion in the Directors' Fortnight at Cannes, where the audience feted Humpday's leading men with a several-minute standing ovation. Last month, Humpday was the Centerpiece Gala at the 2009 Seattle International Film Festival, with Shelton named first runner-up for SIFF's Best Director honors. This weekend, Humpday begins its proper theatrical run, and if you're an appreciator of tiny indie flicks that pack a surprising punch (aesthetic referents: John Cassavetes, Chuck & Buck, Nicole Holofcener, "mumblecore"), you must see Humpday.

As you know, Shelton's film announces itself with a ridiculous premise: A pair of thirtysomething, heterosexual male friends decide to have sex on film and submit the results to an amateur- porn competition. From this iffy point of departure, Shelton spins a small cinematic miracle: a deep, hilarious, completely contemporary relationship comedy that explores with almost scientific precision how such a ridiculous premise would play out in real life. Crucial to Humpday's success is its style, with writer/director Shelton continuing her quest for what she calls "total naturalism." This quest commenced with Shelton's second feature, last year's well-regarded My Effortless Brilliance, in which Shelton found her desired naturalism—there's not a moment in the film that feels forced—though audiences were reminded that sometimes pure naturalism leaves something to be desired. (Many of Brilliance's unforced stretches could've used some directorial forcing.)

With Humpday, Shelton's aim for the tiny moments that hold explosions has improved exponentially, as My Effortless Brilliance's pockets of inspiration are stretched into Humpday's feature-length triumph. The result is a plot-driven film composed of what feel like found moments—a rich trick that's becoming Shelton's stylistic calling card. But how does one reliably create moments that feel found?

"Casting is so important," Shelton told me last month in the SIFF suite at the W Hotel. While Shelton conceives the basic story, she says, "the ideas of characters are fleshed out by actors, and the performers come up with their own words." Preshoot preparation involves "talk, texturizing of characters' backstories, but no rehearsal, just discussion." When it's time to shoot, the actors are set free to do what they do, with director Shelton overseeing the action and letting scenes wander where they may, knowing they can be pruned down to essentials in the editing room.

With actors functioning as de facto screenwriters, casting is crucial, and Shelton wisely placed a well-seasoned mumblecore veteran at the center of the Humpday experiment: Mark Duplass, cocreator and star of 2005's The Puffy Chair and a naturalist actor of extraordinary warmth and skill, whose work Shelton came to love while shooting still photographs of True Adolescents, Craig Johnson's Duplass-led film that also played SIFF 2009. "Watching him act, I was really impressed," says Shelton, who soon after pitched Duplass the idea of a film about two straight guys who decide to have sex. "Originally, I envisioned Mark as the wild one, but he fought to play the domesticated Ben," Shelton tells me. "He'd gotten married and was having a kid, and he was totally feeling that character's story. I told him this required him to help me find someone who could fill the other role and match him on-screen, and he immediately brought up Joshua Leonard, who of course had experience working in this nonacting acting style from The Blair Witch Project."

Humpday's leads carry the majority of the movie, but the heart of the film is Anna, the domesticated Ben's not-so-domesticated wife, played to quiet perfection by Alycia Delmore, a Seattle actress Shelton's been hungering to work with since casting her in a small role in her 2006 directorial debut, We Go Way Back. Another crucial female role in Shelton's bromance is played by Shelton herself, who more than holds her own on-screen and wonders if she'll try it again. "I don't know how people do it," says Shelton of directors who star in their own films. "I love to act, but acting and directing at the same time is insane. It's hard for me to turn off my director's brain."

It's unlikely Shelton will be able to turn off her director's brain anytime soon, as the success of Humpday is opening a variety of doors for the 43-year-old filmmaker. Will Shelton soon be helming summer blockbusters? "It would be foolish to not take advantage of opportunities, bigger budgets, all that," says Shelton, who admits she's fielding a variety of offers. "But my next project is my little movie with Sherman and Sean"—that's Sherman Alexie and Sean Nelson, two Seattle notables with notable ties to The Stranger, whom Shelton will film engaging in a My Dinner with Andre–esque discussion. It's another iffy premise—but after Humpday, Shelton's iffy premises can be seen as the first step toward brilliance.

Do it yourself! HUMP! 5 is coming. Info at theStranger.com/hump