For writer and director Lynn Shelton’s latest, “Your Sister’s Sister,” she enlisted the help of stars Emily Blunt, Mark Duplass and Rosemarie DeWitt — and gave them all creative consultant credits on the heavily improvised project, about a pair of estranged sisters (Blunt and DeWitt) and a male pal (Duplass) stuck in a mourning-fueled emotional ditch. Shot for a scant $80,000 in 12 days on an island outside Seattle, the picture was a bonding experience for the four of them, to put it mildly.

“My No. 1 guidepost when I’m making these films is believability,” Shelton says. “I want to create characters who you really believe in, who feel real and resonate. I really want you to recognize the people onscreen and have them be fully human and therefore fully flawed because we’re all cracked vessels.”

Duplass, who has developed quite the roster of flawed characters in films like Shelton’s “Hump Day” and his own projects with his brother, sees an odd appeal to the less-than-perfect.

“It’s great that you have people identifying with sort of morally and ethically questionable protagonists in this film,” he says. “They’re very human, they’re very flawed, but hopefully what we’ve done is provided in-roads to them so you can identify with those flaws in a way.”

Part of keeping things real meant letting English actress Blunt depict her character as English, and Shelton says there was little question in her mind on the matter. “I didn’t want her to have to worry about that when she was improvising, because improvising was going to be hard enough,” Shelton says.

For Blunt, though, there was more to it than just having one less thing to worry about. “I think the character of Iris kind of lent itself more to being a British person just because she’s not very good at talking about emotions and feelings,” Blunt says. “She procrastinates and is self-deprecating and all of those things, so I think it kind of lent itself more to my kind of humor and the way I probably was. I feel like Iris was me eight years ago. It’s weird.”

Improv Preferred

Duplass found ‘Sister’ to offer a refreshing change from the typical Hollywood style.

“Sometimes in those movies where you have these quote-unquote complex relationships, I find that characters are all behaving pretty well and perfectly, and these characters in this movie are much more like me,” he says. “They have many more moments of social awkwardness and missing the mark quite often — and good intentions at times but really just f—ing up, you know?”