Portraying a single mother whose teenage daughter suddenly disappears in "American Woman" was a "grueling" role for Sienna Miller that forced the British-American actress to imagine her worst nightmare.
Miller plays Debra, a 32-year old supermarket cashier whose daughter Bridget, a mother herself, fails to return home after a night out.
Set in rural Pennsylvania, the harrowing drama follows Debra over the course of 11 years as she looks after Bridget's toddler son and faces personal hardships.
"It was grueling ... The idea of exploring that, the unimaginable, my worst nightmare, felt interesting and that being said when I agreed to do the film I was praying I wouldn't have to because I started to dread what that would be and it was impossibly hard," Miller told Reuters in an interview.
"Obviously not nearly as hard as the actual experience but I spent a lot of time with people who had gone through that and it was crushing. But we were a very strong group of cast members and filmmakers and everybody was very supportive throughout, which I think I really needed."
Miller, who has a young daughter, is known for supporting roles in dramas "American Sniper" and "Foxcatcher" but she said "American Woman" was harder than her past work.
"There were moments where I completely fell apart ... I think anybody would, but I felt supported and I had my daughter to go home to every night which is a huge blessing," she said.
Critics have praised her performance as Debra, who transforms from brusque mistress to a more settled grandmother.
"Women are often left to pick up the pieces of a broken life and struggle through it and do their best," Miller said.
"I liked that it felt feminist, I liked that this woman was messy and imperfect and I love looking at the bravery of a woman like that ... I liked potentially not respecting her at the beginning and really respecting her by the end."
The film, released in Britain on Friday, also stars "Mad Men" actress Christina Hendricks as Debra's sister Katherine and "Breaking Bad" actor Aaron Paul as love interest Chris.
Miller, 37, said the female-led storytelling felt "new and original" as well as "essential".
"I'd never read a script that was one woman in every scene, I never had that experience in the 15 years that I have been making films," she said.
"I feel like I've seen men represented in that way and grown up with that. Women are over half the population, they deserve to be represented as well ... But the industry is shifting in a positive way and so I hope to make more of these."