“What?” I said. “What’s happening?”

The train car rattled. Dern held a finger up, and a small smile spread across her face. She was concentrating, not wanting to miss a whisper. I strained to hear. Was someone talking about her? Had she been recognized? Was there a fight brewing behind us? Her eyes flashed conspiratorially. She was really enjoying this. I, on the other hand, was panicking.

Eventually I figured it out: Dern wasn’t actually eavesdropping on anyone. She was acting out a scene, pretending to be in a restaurant with her kids, listening to the conversation at the next table. Once I understood, I couldn’t stop laughing — the moment had been so all-consuming and strange that I nervously collapsed under its weight. “You can tell your editor that instead of watching my work, we did a scene,” Dern said. “It’s like, I can show you — I can disappear into my characters!”

This impromptu performance had a larger point: What was special about watching her mother rehearse the Preston Jones play was seeing Ladd become as exposed and vulnerable as a real person having a “publicly private moment” in an airport or restaurant. “You know, a Method training would say, ‘You can’t make that,’ ” Dern said. “You are having a publicly private moment, somehow, for you, whatever your process is.”

The Method is strong in Dern’s family. Bruce Dern and Ladd studied at the Actors Studio, and Laura took classes at the Lee Strasberg Theater and Film Institute as a 9-year-old. Dern’s acting teacher of 30 years, Sandra Seacat, also came out of the Method. But Seacat’s pedagogy is a little different. The process begins with the actor writing a letter to herself. According to Seacat, such a letter might read, “Dear Inner Self, if it is your will, please reveal to me in a dream tonight, the inner truth of this character that is in me, this part of me that’s the character.” Then the actor brings the dream to workshop, where she might perform rituals based on it or cast other actors to perform it. Seacat believes, and Dern strongly affirms, that the artist is a “wounded healer” and that by finding in her character her own “wound,” and expressing it, she has the power to heal herself and the audience.

To training, Dern also brings research. In Concord, for instance, she was learning how to roast a chicken over an open fire and reading books from the time period, including the letters of Louisa May Alcott’s mother, Abigail. But all that historical investigation is background for the intensive personal labor of discovering “why I relate to this character, why I’m playing this character, because there’s a reason the character came off the page and I thought, I have to play this.” Once she is in the role, Dern is known to be generous to her fellow actors. Edward Zwick told me a story about Dern playing a scene with her “Trial by Fire” co-star, Jack O’Connell. It was an intense scene for O’Connell, and Dern, even though she was off-camera, was “transcendent,” giving him everything he needed to perform. Zwick feared she wouldn’t be able to reach that height when he had the camera on her. “Then you turn around and see her find it again,” he said. “Her capacity is abundant. She can always find something new.”

Dern isn’t afraid of aging. She says she likes being over 50. “I can, in the same year, be the same age and playing a very sexualized character, a very maternal character, a very heroic character, a bitch, and there’s nothing saying, ‘Oh, now that you’re this age, we’re gonna define you,’ ” she said. Whereas actresses of her mother’s generation turned to theater or projects with foreign directors, now there are, in Hollywood, a “wealth of options — live-streaming, cable, film — to tell women’s stories.” But also, she says, the choices she made earlier in her career, including the choice not to work when there wasn’t a challenging part on offer, have paid off. “That was advice from my parents,” she says — to turn down roles that would pigeonhole or typecast her — “and I resented it. I gave up financial opportunities, I was pissed off a lot, I was bummed because I wasn’t working, but in the long run, I worked with the directors that moved me and I never played the same part twice in a row.”

Growing up, she thought that being an actress would mean “something I haven’t had until now.” She imagined it would involve the kinds of relationships her mother had with women like Shelley Winters and Jane Fonda — “sitting around a fireplace with a group of revolutionaries, talking about art and how the art can make a difference, how to use their voice, how to get in the streets, how to parent, how to single-parent and be an actress, how to navigate relationships and intimacy through all of it.” But until recently she didn’t have the feeling of being in a “tribe” of women. Friendships were isolated. You had dinner with one person or another. You didn’t get together and talk about the industry. You never compared salaries or talked about your deals. Time’s Up and the female-heavy cast of “Big Little Lies” changed all that for Dern. On the set of “Little Women,” she liked nurturing her younger cast members, checking in on them. (According to Gerwig, they all went to her with their problems.) “I like the pack,” she said.