Saul (Peter Sarsgaard) and Sylvia (Jessica Chastain) meet again at a high school reunion. They have a complicated history. Saul lives in Brooklyn and is dealing with the onset of Alzheimer’s. Sylvia, a recovering alcoholic, agrees to be among his caretakers. What follows is a trip to the unexpected.

“Memory” began with Chastain’s pursuit of Mexican director Michel Franco, known for dark portraits of dysfunctional families. “Michel wanted to work in New York and he’s so open to collaboration,” Chastain, 46, said in a Zoom press conference with her co-star. “He asked who I wanted to work with and Peter was my first choice.”

“Michel really wants to get into a city,” Sarsgaard, 52, discovered. “A lot of my process was just taking him around the city. I’ve lived here for 30 years.”

“I love movies that take place in New York and New York is really a character in this,” Chastain noted. “Michel, like his characters, wants to explore the unexpected. Like an apartment over a tire shop where someone lived.”

“If you attach yourself to real life,” Sarsgaard knows, “you find kismet.”

As “Memory” reveals history and the unexpected, so it was as the actors discovered the material for the first time.

“I had the experience of reading the script before meeting Michel. I know he can go quite dark,” Chastain explained. “I got to the scene of Saul and Sylvia in the park and I thought, I know what this is — a revenge film! MeToo! The woman causing havoc on everything.

“As I turned the page I realized Michel goes against every expectation. With each scene I never knew what was going to happen to the characters.

“He likes people,” she added, “to walk in with an open mind and open heart and know nothing about it.”

Franco’s rigorous style is far from Hollywood conventions.

“I look raw in this film which is what Sylvia needs to be. I started out in small plays; you do your own makeup for theater. So it’s not that big a feat to do my own hair in this film.

“I did this after the Oscar” – for playing Tammy Faye Bakker – “and people said to Michel, ‘She’s going to leave your film.’ But sometimes the more money or trailers you have, it slows everything down.

“When it’s a smaller budget it feels more collaborative in some sense. The first day was a real AA meeting — and I was not prepared for that.

“But working this way it forces you to be a human being and present as a character without ‘acting.’ ”

“Memory” opens Jan. 5