Director Lynn’s Shelton’s love-triangle-with-a-twist dramedy Your Sister’s Sister (which opens in select cinemas nationwide this week) is the best kind of micro budget American indie: a thoughtful, engaging, and, in its own unprepossessing way, daring feature built on strong performances and interesting characters.

After making a scene at a memorial party for his late brother Tom, Jack (Mark Duplass), still struggling to come to terms with his death, is invited by best friend Iris (Emily Blunt) to spend time at her family’s holiday cabin. Shortly after arriving he meets Iris’ sister Hannah (Rosemarie DeWitt), who has just come out of a long term relationship. They get drunk, sleep together and awake to find Iris unexpectedly arrive, setting off a chain of dramatic events in which everybody’s true motives and desires are eventually revealed.

One of my first reactions to Your Sister’s Sister was to laud Shelton’s screenplay (she is credited as writer) as a measured, mature, naturalistic and tightly constructed work. So I was surprised to have my first question in this interview with Mark Duplass, who is terrific as Jack, politely rebutted.

“There was no script,” he told me. “All the dialogue from every scene was improvised.”

Duplass explained to me how this minimalistic and quietly affecting film came to be. However, he kept mum when quizzed about his role in Kathryn Bigelow’s highly anticipated Zero Dark Thirty, about the hunt for and death of Osama bin Laden.

“I wish I could (tell you about it),” he says, “but we are keeping all the details under wraps.”

Firstly, congratulations on your performance.The cast in Your Sister’s Sister obviously needed to give it their all to make these characters work. When you started reading the script, what were your first impressions of your character and how did those impressions change as you kept reading?

I actually brought the story idea to Lynn Shelton, with whom I had worked with on Humpday. And from there we made a detailed outline along with the other actresses. So, there was no script for the film. All the dialogue from every scene was improvised from the outlines.

Your performance is able to pull off lines like “your butt is super soft” without sounding sleazy. Did you worry that a line like this would sound a bit dirty?

Again, hard to say, because it just came out in the heat of the improvisation. But I said it with love and admiration, so I hoped I could pull it off!

For almost all of Your Sister’s Sister there are only three characters. The dynamic between them is intimate but fractured. What was it about the central premise that most appealed to you?

I like the idea that you can have a comedy with three people who are in inherently dramatic situations. These people are really messed up, but you can still laugh with them and enjoy the process of them fumbling their way toward happiness.

With only three actors on set, can you reflect for me on the preparation and shooting process? The characters are quite nuanced. I imagine that you, Emily (Blunt) and Rosemarie (DeWitt) spent a lot of time rehearsing beforehand, is that right?

There were no rehearsals. I really believe for films that use improve, you can blow a lot in rehearsals. We do, however, build a large back story from which to draw the improv. That took place over many phone calls in the months before we started shooting. The shoot itself, though, was less than two weeks. Eleven days, I think.

With such a tight focus to Your Sister’s Sister, there is very little that Shelton could do in terms of cutting around you, or finding space in between performances. It came off very smoothly but did you ever worry, or at least consider, that it might not work? If one part of the equation fell down — for example, if one performance wasn’t that great — then everything could have potentially tumbled down with it.

Definitely. I’m always worried about that but in the case of this film, it’s very easy to fail. It’s just three people and a house. That’s it. If we don’t have chemistry, the film is a dud.

“in the case of this film, it’s very easy to fail. It’s just three people and a house.”

Did you find that there was more time to “get it right” on Your Sister’s Sister, or was it the opposite way around, i.e., a small budget and a tight shoot?

It was small and short, but we had the time we needed. We don’t futz too much with lighting and technical stuff on these films, so we spend most of our time acting and trying to get the scenes right from a performance standpoint.

Your Sister’s Sister is a great little micro budget indie: thoughtful, naturalistic and built on interesting interpersonal relationships. What other very small budget indie films are you particularly fond of?

I loved Oly Boy, by Kelly Reichardt. And there’s a film called Prince of Broadway made for peanuts that knocked my socks off!

One of your upcoming films is Kathryn Bigelow’s man for bin Laden flick Zero Dark Thirty. Can you explain what kind of character you have in that film, and, broadly, what the shoot was like?

I wish I could, but we are keeping all the details under wraps. It’s gonna be the film of the year, though. Amazing stuff.

The acting industry, as you well know I’m sure, is fiercely competitive. Are you at a point in your acting career where you can pick and choose your roles or is that sort of thinking a pipe dream?

I have it very good as an actor. And while everyone always wants a little more access and a little more say in things, I am now in a place where people know me and I get the chance to do great films. That’s more than I ever thought I’d achieve.

Your Sister’s Sister’s Australian theatrical release date: September 6, 2012.