NEW YORK – When Jessica Chastain signed on for “Memory,” she knew exactly the kind of film she didn’t want to make.

Michel Franco’s quiet new drama (in theaters now) follows the uneasy bond between Saul (Peter Sarsgaard), a man with early-onset dementia, and Sylvia (Chastain), a single mother and recovering alcoholic. The film is tender yet refreshingly unsentimental, forcing audiences to reckon with moral gray areas and unanswered questions as Sylvia confronts her past sexual assault.

“On the first day, you were like, ‘I just don’t want this to be Hollywood cupcake,’ ” Sarsgaard recalls to Chastain, seated together at a Chelsea TV studio. “That phrase was in my head for a while, trying to imagine what that version of this movie would be.”

“Oh, my God, that’s so silly,” Chastain says with a laugh. “But I probably said it in a very Sylvia way: very serious, ‘I don’t want this to be a Hollywood cupcake movie about dementia.’ ”

Peter Sarsgaard looked to his late uncle to play 'Memory' character with dementia

When we first meet Saul, he follows Sylvia home from their high school reunion. She eventually corners him, accusing him and a group of boys of raping her when they were teenagers. Saul insists he wasn't there, and when Sylvia learns he’s telling the truth, she volunteers to become his caregiver.

To play Sylvia, “I really had to make as specific as possible the memories that she’s trying to run away from,” says Chastain, 46. “What happened before she decided to become sober? How was her daughter conceived? She just wants to get through the day and go to sleep with her daughter safely in the other room. And then Saul, in some sense, switches that all up for her.”

As Saul and Sylvia spend more time together, their friendship turns romantic. Because of his dementia, he can’t remember their everyday conversations, trips to the local diner, or nights snuggled on the couch watching movies. Saul offers Sylvia the perfect blank slate: giving her a shoulder to cry on when she needs it most while also never letting her assault define her.

“The beauty of this relationship is somebody who can’t forget this thing that happened in her past, hooking up with a guy who sees her new every day,” says Sarsgaard, 52. “ ‘You’re not this person with trauma.’ I think that’s the dynamic that people respond to when they see this movie.”

Sarsgaard, who won best actor at Venice Film Festival for his performance, was partly inspired by his uncle Bubba, who had dementia and died recently. Though many view dementia as taking away parts of one's personality, Sarsgaard wanted to show that's not always the case. Like his uncle, Saul radiates positivity even as he grapples with the disease.

"My uncle was in a nursing home in Tennessee, and we sang this Kermit the Frog song to him over the phone, 'Rainbow Connection,' " the actor says. "I just remember him laughing. I don't know who he thought I was at that point, but it's this basic idea he had of: 'This is my friend. These are the people I know and I trust this person.' So I really thought of that when I was playing this character.

"They're still who they were yesterday; they're not different now that they've been labeled with dementia."

Jessica Chastain bought her own costumes at Target to help create Sylvia

"Memory" marks a return to Chastain's independent roots, after her Emmy-nominated turn in Showtime's "George & Tammy" and Oscar-winning role in "The Eyes of Tammy Faye." The actress has appeared in multiple blockbusters over the years, including "The Martian," "Interstellar" and "It: Chapter Two." But it was her breakout roles in low-budget dramas "Jolene" and "Take Shelter" that first grabbed Hollywood's attention.

"I'm very comfortable working without a trailer (on the set)," Chastain says. "Michel was a little bit afraid to work with a movie star, perhaps, because people had said: 'Oh, she just won an Oscar! She's going to be a nightmare! She has all these fancy things people do around her!' So it was joyful for me to go to Target to buy my costumes, because I also got to participate in making Sylvia. I always say, if my character can get dressed by herself, I don't need a dresser."

"Now if you're playing Marie Antoinette, then you get a dresser," Sarsgaard quips.

"Yeah, if I'm playing Marie Antoinette, watch out, crew!" Chastain jokes. "I'm having six people around me at all times!"

For both actors, making the movie brought back memories of other scrappy shoots when they worked on the fly. Sarsgaard recalls one early project, "In God's Hands," when the cast changed costumes in a van. The project was subsequently shelved after filmmakers realized the entire thing was filmed out of focus. Chastain got similarly resourceful on Terrence Malick's "The Tree of Life," staying in a house just across the street from the set and choosing her own wardrobe. The shot where a butterfly lands on her hand was unplanned.

"I was in costume and he just said: 'Go! Chase the butterfly!' " she says.

Chastain can't bring herself to revisit the movie, in which she plays a mother who leads with grace in the face of loss. She last watched it at the 2011 Cannes Film Festival, where "I wept the whole time."

"The little boys have grown up, so they're men now," Chastain says. "Terry saw a part of me that I didn't even know existed. So I just don't know how to watch it and not be incredibly affected. I just need time."

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