FILMMAKER Lynn Shelton knows all about working on a small scale, but she is also very certain about the possibilities it offers. Your Sister's Sister is, she says cheerfully, the third film she has directed that is basically ''one location with three characters''. It's also a movie made in the way she loves to work: in close collaboration with her actors. They are intimately involved in the development of their characters, and that's why her three leads are credited as ''creative consultants''.
The film begins with a wonderful, disconcerting scene at a wake. It's where we meet Jack (Mark Duplass), whose brother has died and whose response to the expressions of mourning takes people by surprise. Also present is Iris (Emily Blunt), the former girlfriend of the deceased.
She sends the numbed Jack to her family cabin in the Pacific north-west, where he can have time to himself. She plans to drop in on him. Neither is aware that Iris's half-sister, Hannah (Rosemarie DeWitt), will also be there, wrestling with uncertainties of her own. From this emerges a tale of alliances, expectations and observations, with some thoroughly surprising twists, which are comic, melancholy, meandering, poignant and awkward, often all at once.
It was Duplass who came to Shelton with the germ of the story, although his idea involved a mother and daughter, rather than two sisters. He had starred in her previous film, the equally lo-fi and confined Humpday, about two straight male friends who decide to make a gay porn movie together.
Until the last minute, the sisters were to be played by Blunt and Rachel Weisz. Then scheduling conflicts arose and Weisz had to withdraw.
Shelton was thrown at first. Everything was locked in for the shoot, she says, and she didn't have any leeway. Replacing Weisz at short notice didn't seem possible ''because I had built this character with and for her''. But when she found DeWitt, the extensive preparation meant she could offer her something solid on which to build. In a short time, Shelton says, ''we had to figure out how to get her stamp on the character''.
DeWitt - who played Don Draper's artist girlfriend Midge in Mad Men and Toni Collette's sister in United States of Tara - underwent intense preparation. She was handed what Shelton calls ''a scriptment'' - halfway between a script and a treatment - and a ''backstory bible; the story of these three people's lives up to the moment of the film taking place''.
Shelton, like director, writer and actor Duplass, is associated with a low-budget filmmaking aesthetic known as ''mumblecore'', which includes a commitment to improvisation and a sense of heightened naturalism. She wants her films to convey the feeling that the characters exist before and after the events onscreen.
Duplass is an effortless, inventive improviser, Shelton says. And Blunt's breakthrough film, Pawel Pawlikowski's My Summer of Love (2004), about a relationship between two teenage girls from different backgrounds, was based on improvisation. ''It was a film she loved working on,'' Shelton says. ''She had great memories of it and she wanted to work that way again.''
Improvisation, for her, involves certainties and possibilities, and ''having a map of the emotional dynamics of a scene''. The actors understand what the scene needs to convey, but have the freedom to explore ways of reaching it, ''and I'm the one that gets to pick''.
To Shelton, asking actors to plunge into improvisation means she prefers they speak in their own voices as much as possible - that they don't assume an accent, at any rate. Weisz and Blunt are English, but DeWitt is a New Yorker, and audiences might have been distracted by the fact that the sisters appear to come from different countries. However, Shelton says one of her earlier decisions turned out to be serendipitous. When she cast Weisz and Blunt, she decided that the characters were half-sisters. They are close, but there's a history of friction from childhood tensions about their parents' break-up. This point of difference, she says, became even clearer, without being unduly emphasised, when DeWitt came on-board.
Just before Your Sister's Sister, Shelton took a very different kind of creative assignment: she directed an episode of Mad Men, titled ''Hands and Knees''. It was a fairly momentous instalment that involved pregnancy, a panic attack and a visit to the Playboy Club. For Shelton, it was a wonderful experience that seemed utterly new, yet surprisingly familiar. ''I couldn't believe that I was working on my favourite TV show and I still pinch myself that I got the opportunity.
''It was my first time with a union crew, on a sound stage, in Los Angeles. So many new things, and yet I discovered that it was still exactly the same job.'' The scale was different: instead of a production designer, she had a team. ''We were making the same kinds of decisions about what was in the frame and how we covered the scene,'' she says. ''But in TV, as a director, you're trying to channel the creator's vision. I'm the captain of the ship, not the admiral of the fleet.''
She also directed an episode of the Zooey Deschanel sitcom New Girl and is set to do a couple more of them, as well as a new Fox show, Ben and Kate, from the producer of New Girl. She had wanted to do TV, she says, ''because I thought it would be a nice way to continue to make the kinds of movies that I really want to make''.
For now, she is in the editing suite working on a film called Touchy Feely, which she shot in April and May. It has a bigger cast; as well as working with DeWitt again, it also stars Ellen Page, Allison Janney, Josh Pais, Ron Livingston and Scoot McNairy. It's something different, she says: ''I was shooting on a tripod, ensemble cast and multiple storylines and it's really more of a drama. It has some laughs, but you wouldn't call it a comedy by any stretch of the imagination.''
Even then, she found herself ''fantasising about going back to the bare bones of three people and a room''.
■Your Sister's Sister opens September 6.