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Brace yourself: The bloodstained suspense comedy “Thoroughbreds” slays every belief we have about good breeding.

It follows Amanda and Lily, perfectly turned-out teen girls living in an East Coast community of imposing mansions, elite private schools and private equestrian stables. Though the former childhood friends have facades of calm poise, they are ostracized by the upper-crust community for scandalous misbehavior, one involving lies, the other, bloodshed. The pair reunite, weaving their knotty relationship into a noose as they plan to secretly murder Lily’s infinitely annoying stepfather.

Adapting one of his own stage works into cinema, playwright-turned-director Cory Finley has spent his time since 2010 “either off-Broadway or off-off-off-Broadway, doing small 80-seat theater sorts of plays,” he said in a phone conversation last week.

Among those plays, “Thoroughbreds” held Finley’s attention strongest because “sort of late in the process of putting it together, it started feeling increasingly like a movie.” He wanted to direct it himself because screenwriters on independent films don’t have the degree of authority over production of their work that playwrights enjoy. Since its debut in last year’s Sundance Film Festival, “it’s been an incredible ride in a lot of ways, and a true surprise, and a joy.”

While he hadn’t had a deliberate strategy to cross into film, his ambitions “were somewhere in between. A lot of my play-writing and theater peers were surprised to hear not that it was a movie but that I directed it myself.”

While other playwrights were moving on to television writing, “I always loved stories that had a clear beginning, middle and end.” With “Thoroughbreds,” he saw “a tiny window of opportunity that opened itself up earlier than expected and jumped on it. I didn’t really think it would all come together until it did.”

Finding the cast was the first order of business. He said, “We got a cast that were universally down to have fun and very, very collaborative.”

Finley was “a huge fan of both” Olivia Cooke (“Me and Earl and the Dying Girl”) and Anya Taylor-Joy (“The Witch”), who star as Amanda and Lily. To function as more than an eerie two-person relationship drama, the film needed a third wheel in the form of a comedic henchman. The late Anton Yelchin (“Star Trek: Into the Darkness”) plays the duo’s foil, a struggling class drug dealer with a degree of empathy that makes him see the world very differently from his “partners in crime,” Finley said. “It’s one of his huge gifts that you always care so much about his characters and what they’re going through. His ability to draw your sympathy was incredible.”

The social setting of the story contrasts the lifestyles of the leisure class with a down-on-his-luck striver who sees a great growth market in narcotics. “I think for better or worse, a clash of economic strata has been a theme in a lot of my writing,” Finley said. “I have very complicated feelings about wealth, and they often end up surfacing in a lot of what I’ve written.”

Another recurring theme in his work is joking about things that are dead serious. “I’m always attracted to mash-ups of tone, and to very dark humor. Anxious, uncomfortable humor. And I see ‘Thoroughbreds’ as a sort of unholy child of a psychological thriller and a dark comedy.”

In his mash-ups of genre conventions, he’s following in some well-known footsteps.

“Some of the filmmakers who made me want to get into that business were the Coen brothers, who have such a mastery of that tone. Paul Thomas Anderson is another, a constructer of those stories in their own little world. David Lynch’s movies opened up my mind about how much you can accomplish with sound design.”

Finley, 29, knew he required expert guidance to achieve the technical polish he wanted for the film. Cinematographer Lyle Vincent (“A Girl Walks Home Alone at Night”) gave the centerpiece location, a mansion with exceptionally wide and long corridors, the sort of slithering camerawork that gave “The Shining” its haunting tone. And the sound crew gives a sense of alarming life to invisible things only heard from off-screen.

“I was very careful about picking people whose work was a real tonal match with what I wanted to do,” he said. “One of the real joys of wrapping my mind around filmmaking is that a director has so many more tools, so much more control over tone than a playwright or a stage actor does. You can’t move a camera or have control over where an audience is looking as you can here.”

Finley said he is “very carefully” approaching how to follow up a debut film that could be interpreted as a massive case of beginner’s luck.

“I want to make movies that are unique and that express a particular sensibility about characters to care about. I’m always interested in stuff that has a little bit of comedy but not broad comedy and characters that I can identify with at a deep level.

“We can’t get too into this but I definitely indentify uncomfortably deeply with both of the leads of ‘Thoroughbreds,’” who are beset by conflicting feelings of joy, terror and hysteria. “I think at the root, that’s where the story came from. They’re heightened versions of things that were troubling me about myself,” he said.

“Not that I’m going to go kill anyone.”