06/13/2012 08:31 am ET Updated Aug 13, 2012

Lynn Shelton Makes It Up As She Goes Along

It wasn't supposed to be a sister.

In fact, as originally conceived by writer-director Lynn Shelton, Your Sister's Sister could have been titled "Your Sister's Mother."

Her new film, opening in limited release Friday (6/15/12), tells the story of a lonely guy who sleeps with his best female friend's sister without realizing the pair are related. But as Shelton was first presented it, the character was the friend's mother.

"He met his friend's mother and slept with her," Shelton says with a laugh during a telephone interview. "But we thought that would be too Oedipal -- to have a mother-daughter love triangle. So we changed it to sisters right away."

Your Sister's Sister, a hit at the Toronto Film Festival last fall, is Shelton's follow-up to her 2009 indie hit Humpday. It reteams her with Mark Duplass, a Humpday star who plays the guy at the center of the triangle.

"Mark came to me with this idea that he and his brother (Jay) had for a film about a guy who's lost his brother," Shelton says. "But they thought it was too close to home for them to do themselves. But he liked the idea and he called me. I liked the plot -- although there was no sister involved in that first iteration; it was the mother. And Mark and I had been looking for something to collaborate on."

Duplass plays a guy still grieving the death of his brother a year after the fact. Emily Blunt is his best friend Iris (who happened to date his late brother), who offers the use of a family cabin on an island off Seattle for him to get himself back together. Thinking he'll be alone there, he arrives to find someone else at the cabin: a woman named Hannah (Rosemarie DeWitt), with whom he sleeps and who turns out to be Iris' half-sister.

Shelton works in an improvisational manner. She writes an outline of scenes, then lets the actors improvise their lines as they create the scene. Still, she worked in a slightly more regimented way this time.

This interview continues on my website.