Since Lynn Shelton’s highly improvisational 2009 comedy "Humpday" broke through the fest-circuit glass ceiling, her stock has risen, and she's directed episodes of "Mad Men" and "New Girl." Now, instead of going full Hollywood, she’s bringing the stars to her.

In Shelton's latest film, "Your Sister's Sister," Emily Blunt joins Rosemarie DeWitt and Mark Duplass for a low-budget relationship comedy that, like "Humpday," was heavily improvised on set. Blunt plays Iris, who invites her best friend (Duplass) to stay at her family's remote cabin as he grieves over the death of his brother. When he gets there, he finds that her sister Hannah (DeWitt) is also there. That's when the fun begins.

Here Shelton, Blunt and DeWitt chat about the challenges of improvisation and the remarkable nature of family ties.

Lynn, how did this story come about?

Lynn Shelton: I've always had girlfriends. I can think of two or three who have the most unbelievably fascinating sibling relationships with their sisters. Just so many layers of this connection, this need to feel connected and bonded with each other, and then all kinds of stuff underneath -- whether it’s resentments from past betrayals or just the way that they seem to be able to hurt each other more than anyone else on Earth. And I just thought it would be an interesting territory to explore.

Was Emily one of your top choices to play Iris?

Shelton: Emily was very present for me. She was definitely in my top choice because I was kind of obsessed with her.

Emily, were you familiar with Lynn's style of making movies before signing on?

Emily Blunt: Yeah. I had seen "Humpday" and my agent called me and said Lynn Shelton wants to give you a call ... and I was so excited to speak to her. I could see in "Humpday" how incredibly fresh and spontaneous it felt and how true to life it was. I felt so connected to what was happening. And I had been crying out to work like that for a while.

Rosemarie DeWitt: [laughs]

Blunt: What?

DeWitt: You were crying out, like, aahhh!

Blunt: [laughs] Who will work with me?! Yes, so my cries were answered. Finally.

Rachel Weisz was originally supposed to play Hannah, right?

Shelton: Yeah. She had to drop out. When it happened I just felt, the movie is no more. It was gone. And my co-producer started to put out there, "Well, what if we could find somebody? We might as well try?" And I was like, "No. No. It’s never going to happen." It was only three days before we were supposed to start. And they started putting names out, but when I thought of Rose I thought, oh, maybe she could do it. And when I mentioned her name to Mark [Duplass] he was so sure of it. He said, "If she’s available, she’ll do it."

DeWitt: I accosted him at the airport in New Orleans after I saw "Humpday." I was just like, "Oh my God, that movie is amazing, I want to work with you." And I never do that.

Shelton: And that was the thing, we knew that she had seen "Humpday" and it appealed to her. We had no time, so we had to find somebody who was familiar with my work.

Rosemarie and Emily, can doing this amount of improvisation become overwhelming?

DeWitt: Our minds were like sand at the end of the day.

Blunt: Improvising exposition is tough.

DeWitt: And making it immediate is tough. But then finding new moments is great and having Lynn being -- stealing an Emily word -- "conductor" of it all.

Blunt: It was a shape-shifting process and you didn't really know what to expect from each scene, and there's something really exhilarating working like that. But it's quite daunting initially, because you do feel such a great responsibility as an actor because you are writing it. It's falling out of your mouth as it occurs to you, so the immediacy of it ... you can capture golden moments, and you can also feel like you're watching paint dry with some of it.

Compare this to working on a comedy like "The Five-Year Engagement." Is it completely different?

Blunt: To be honest, it's not, because "Five-Year" had a lot of improv as well. The script was much more structured and the scenes were really juicy and really funny, and sometimes we'd play a scene out completely scripted. Other times there would be a couple of lines we'd improv and others we'd go completely off into another area and that, in itself, felt familiar to this. I remember when I signed on it was in its embryonic stages, and [director] Nick [Stoller] called me and said, "Don't base the movie on this, it's going to change and evolve." And as soon as I signed on they invited me to a writers' meeting and I just brought all of my notes and all of my ideas, and they did a complete rewrite for my character, and that's why the characters are so juicy and real.

And with "Your Sister's Sister," it's really the entire movie you have to do that, so that must get overwhelming.

Blunt: But we had the safety net of Lynn being so intuitive and knowing -- as an editor -- what worked, and taking the good bits and letting us feel safe enough to let us just throw everything we had against the wall, and she was going to find the stuff that would stick.

I have no segue into this, so I’ll just say it: Tell me about the "trim your bush" scene.

Shelton: I wanted the dinner scene to have alliances be built.

Blunt: Tensions to rise.

Shelton: So at one point I just told Rose to say something that will really embarrass Emily.

Blunt: The story actually happened to a friend of Rose's. So she remembered the story as soon as Lynn said that. It worked. [laughs] But I feel so embarrassed for whoever that girl is.

DeWitt: It was just some girl that I'm not even friends with, and some lacrosse player told her, "You have a poof in your underwear, did you ever think about trimming it?"

Blunt: Oh, how embarrassing.

DeWitt: So that relayed over to my girlfriends and got to me, and for some reason that flashed into my head.