Director Lynn Shelton has quietly staked out her place at the forefront of the indie film scene, gaining acclaim with “Humpday” and “My Effortless Brilliance.” Her new film, “Your Sister’s Sister,” a comic drama about an unusual love triangle starring Mark Duplass, Emily Blunt and Rosemarie DeWitt, arrives at Portland’s Nickelodeon Cinema on July 13.

We spoke by phone about how naturalistic actors can sell an outlandish premise and why you shouldn’t know too much about “Your Sister’s Sister” before seeing it.

Both “Humpday” and “Your Sister’s Sister” have plots precipitated by, shall we say, unusual events. Is it easier or harder for audiences to accept an unexpected premise in a movie with such naturalistic performances?

I came up with this concept that seemed so out there in “Humpday,” (co-stars) Mark (Duplass) and Josh (Leonard) were highly skeptical that it could be done in a believable way. We all were determined that we didn’t want the movie to be unbelievable at any point, that we wanted the audience with us at every step.

Improv helps the believability factor too. Actors bring their own words, their own cadences — I don’t know how 30-year-old dudes talk when they’re alone. Plus, I love that collaborative process: We all agree that we we’re making the same movie, and then it can be organic and collaborative. Everyone brings all their knowledge and experience to the fore.

Is the combination of an unusual premise and a character-driven story twice as easy or twice as hard to get people interested?

I do a terrible job of selling my movies! I sort of depend on the word of mouth from people who’ve liked my other films. Plus, a hard thing about pitching “My Sister’s Sister” is that every review spoils something. Ideally, learning about these little twists and surprises all add up to a bigger payoff at the end. The audience becomes complicit, and hopefully by the end, they’re invested in how things will turn out.

With such a collaborative process, how did you cope with losing your original Hannah, Rachel Weisz, just days before filming?

We had worked for nine months with Rachel, and I thought we were dead in the water. If you develop a character with an actor, you think it can’t be anyone else. But Rose (DeWitt of “Mad Men”) leapt to the top of our wish list, and I knew if we could get her in the movie, we were gonna be OK. She didn’t have the luxury of those nine months, but she hit the ground thinking.

I really liked “Your Sister’s Sister” a lot. Without spoiling anything, what would you say you want people to take away from it?

We’re all of us cracked vessels, and we all deserve a chance at redemption and forgiveness. It’s not a new story, but you do have control over how it’s told. I’m hoping to tell stories that really resonate, about real people, not sanded-down Hollywood stand-ins. To create a story that’s believable but unexpected.

Dennis Perkins is a Portland freelance writer.