“She understands, in her bones, the culture of that area and that time period,” Baker says. “There were so many conversations we didn’t need to have because of [that] deep understanding.”

Baker has previously depicted New England life onstage, but this is the first time the 43-year-old playwright — who won a Pulitzer Prize in 2014 for “The Flick,” about low-wage workers at a movie theater — has written and directed for film. The recently released coming-of-age drama “Janet Planet” is understated; Baker elevates the simple joys and existential conundrums of everyday life. “Her characters grapple with longings and a need to prove their worth, but they rarely share their struggles out loud,” says The Washington Post’s four-star review.

Thankfully, Nicholson, 52, is known to convey great emotion with little fuss. Her characters’ reticence could be mistaken for stoicism, but those who pay closer attention will detect a whirl of feelings churning beneath the surface. Her furrowed brow speaks volumes. This ability won her an Emmy Award in 2021 for the HBO murder mystery “Mare of Easttown,” in which she portrays a small-town woman who discovers agonizing truths about her family, and caught Baker’s eye in the 2019 film “Monos,” in which Nicholson plays an engineer held hostage in a Colombian jungle by teenage guerrillas.

Nicholson, according to Baker, is an “unshowy actor.”

“She’s always thinking thoughts, and you can feel them on her face, but what exactly she’s thinking is hard to articulate,” Baker says. “That kind of active thinking, combined with psychological mystery, felt really, really important for the part. It’s a very rare thing that she has in spades.”

“Janet Planet” unfolds from the perspective of 11-year-old Lacy (Zoe Ziegler), a quiet, contemplative child who lives with her single mother, an acupuncturist. Janet receives three visitors throughout the summer of 1991 whose stays reveal to Lacy that Janet might not be the woman she believed her to be. Press materials describe this gradual disillusionment as “falling out of love with your mother,” a phrase that now makes Baker wince. “I feel like it’s more complicated than that,” she says.

Once again, Nicholson got it. Lacy comes to see Janet not just as her diligent caretaker but also as a full human being with her own needs. The actress returned to moments in her childhood to better connect with the story, recalling the “long hugs” some of her herbalist mother’s friends used to share. The extended embraces made Nicholson squirm at the time. It was difficult to understand why these adults, who were supposed to have it all together, were clinging to each other.

“You could pick up on the need, or the desperation, from one or both of the people,” she says.

Upon reflection, Nicholson believes her mother’s friends, who led a rather bohemian lifestyle, were “searching for meaning.” In the film, it is Lacy who makes this discovery. She silently observes as Janet embraces her old friend Regina (Sophie Okonedo), estranged since a falling-out years ago. Lacy becomes protective of Janet when her boyfriend, Wayne (Will Patton), lashes out amid his mental health struggles. Later, Lacy watches inquisitively as Janet converses with Avi (Elias Koteas), the leader of a cultish theatrical troupe who sets his sights on her.

At times, Janet invites Lacy into her thought process. When Wayne’s episodes begin, Janet asks the preteen what she should do. Nicholson, recalling the brief period before her own mother remarried, says single moms and daughters share “a very particular relationship.” There can be a level of codependence, she explains, “and I don’t mean that in a negative way. It’s just different.”

Baker, who grew up with a divorced mother, adds that “marriage gets a lot of gray-area screen time without too much moralizing. I felt like I hadn’t quite seen that the way I wanted to in a relationship between a parent and child.”

Over the course of the summer, Janet and Lacy evolve alongside each other. Lacy, who struggles to make friends her own age, tries to make sense of human connection. Her journey is “about how one child’s perception, worldview, philosophy and spiritual life can change over two months, which is an enormous duration for an 11-year-old girl,” Baker says. Janet learns she needs to seek approval from within, rather than from men like Wayne and Avi.

“It’s a huge key into who she is and where she’s been getting a large chunk of validation for most of her life,” Nicholson says. “That kind of attention has always been one she could fall back on.”

Baker says she has written several screenplays throughout her career but selected “Janet Planet” as her directorial debut because “it was the first time I had written a screenplay that I could just really see.” While the story explores universal themes, its casings are regionally specific. Baker returned to Western Massachusetts while writing, spending a month among other artists in the foothills of the Berkshires. She snuck onto the grounds of her old sleepaway camp, Shire Village in Cummington — where they shot the opening scenes of “Janet Planet” — and soaked in the natural sounds of summer in Amherst. The film forgoes a traditional instrumental score so as not to “undermine the power of the bugs and trees,” she says. “I did not want to telegraph at all what was happening inside of [Lacy’s] brain through music.”

Working on the film brought back a flood of memories for Nicholson, who hadn’t been back to the area for roughly 30 years. “It’s insane how much of it is unchanged,” she says. They shot a scene at the Hampshire Mall in Hadley, which still houses that personally pivotal J.C. Penney. En route to another location, they drove through Goshen, where Nicholson was a camp counselor every summer. She brought her family members out, showing her two teenagers the street where she grew up. She visited old swimming holes with her now 73-year-old mother.

Much of the filmmaking process came down to “deepening our connections to the place, because it is such a character in the film,” according to Nicholson. Stepping into Janet’s life — in a setting so personal — prompted the actress to reflect on what it means to investigate where you come from. Was she revisiting the past or creating a new present? Whose inner life was she exploring?

“Is it myself? Is it my mom? Is it my mom’s friends? Women in general? Middle age?” she says. “That involves going deep into your own heart a little bit. But, to me, that’s the juicy stuff.”