The problem with most screenplays, line-to-line and character-to-character, is a problem of differentiation. As in, everybody sounds like the same type of person. Human or human-adjacent qualities, optional. Separate from this problem is the rarified, repeat-Oscar winner realm of screenwriting, where Quentin Tarantino holds court and the writing becomes so self-consciously embroidered that scenes have a way of slowing to a crawl while the writer dog-paddles around for a while. And then somebody shoots somebody.

But there are exceptions to these ridiculous and untenable generalizations. Some of them are even playwrights, breaking into what is, for them, a new medium.

Take Annie Baker, a terrific playwright whose Pulitzer Prize-winner, “The Flick” (2014), served up a luxuriantly naturalistic slice of life, set in a struggling art-house movie theater. Baker has now made her feature film debut as writer-director with “Janet Planet,” and there’s so much right with it, beginning and ending with how Baker listens to, and frames, what her characters say, and how. And what they don’t.

It’s set in early 1990s western Massachusetts, where Baker grew up, in the seldom-filmed Pioneer Valley region. This is a verdant patch of mostly comforting isolation, as Baker remembers it by way of the soundtrack, full of birdsong and insect buzz and the wind blowing through the back seat of an un-air-conditioned car.

At the start, Lacy, 11 years old, sneaks a late-night call home from summer camp. She wants out. Her acupuncturist mother, Janet, retrieves her from camp and gets a partial refund. “Janet Planet” chronicles the rest of their summer at home, as Janet navigates her current boyfriend, Wayne; her old and somewhat bossy actor friend, Regina, whom Janet and Lacy reconnect with at a performance of Regina’s cultlike theater troupe; and Avi, the theater director, who takes a stealthy interest in Janet because, as daughter Lacy says forthrightly, everyone’s always falling in love with her.

How does this play out? In bracingly lifelike exchanges of precise, concise small talk, polite evasions, and occasional, surprising connections, mostly between mother and daughter. They’re close, and there’s a world of love there, no doubt. But Baker isn’t interested in warming hearts the easy way, or in the usual catharsis business. “Janet Planet” is told largely from the perspective of Lacy, a loner, a dreamer, and a very, very close observer of everything to do with her mother.

“Tell me what to do,” Janet says to her, after broaching the subject of problematic boyfriend Wayne. Pause. “I think you should break up with him,” Lacy says. Clearly it’s the right call. It is also the latest of many exchanges between them that underlines a codependency Lacy appreciates, and that Janet knows isn’t necessarily for the best. If it’s about any one thing, Baker’s film is about how Lacy comes to a different place regarding her mother’s dreamy roundelay, distilled in a single image at the very end.

Often “Janet Planet” is a movie of relatively few words. Baker and her cinematographer Maria von Hausswolff allow the environments, indoor and outdoor, do much of the the communicating. A lot of the emotional information reveals itself in ultra-tight closeups of faces, an earring, a forearm, in soft, pastel light. Lacy is trying to learn what it means to be an adult, or at least what it means to be her mother. Baker teases out the everyday, ordinary sound of relationships under duress, as Janet’s house accommodates one outsider, then another, and another.

Late in the film (no spoilers here; it’s not that kind of story) Janet makes a sort of confession to her daughter. “I’ve always had this knowledge deep inside of me that I could make any man fall in love with me, if I really tried,” she tells her. “And I think maybe it’s ruined my life.” It’s a superbly phrased line, saying so much about so much. It sounds like life, not the movies, and in Julianne Nicholson (Emmy winner for “Mare of Easttown”), Baker has the best possible Janet, working with the least possible external effort. She’s a marvelous actor.

As Lacy, Zoe Ziegler matches Nicholson heartbeat for heartbeat, though she’s often filmed alone in her room, playing with her figurines, staging scenes on a miniature stage complete with curtain. The other roles are exquisitely well cast, with Sophie Okonedo (Regina, Janet’s rather needy old friend), Will Patton (Wayne, the problematic boyfriend) and Elias Koteas (as Avi, the puppet-theater guru with an extreme way of pausing before speaking). The film is a mite thin, and occasionally glib. But Baker knows where the bittersweet human comedy lies in this mother, and this daughter. Those in need of conventional uplift in their family stories, or more assertive character arcs, never have to wait very long for what they require. Meantime there’s this, for the rest of us.

“Janet Planet” — 3.5 stars (out of 4)

MPA rating: PG-13 (for brief strong language, some drug use and thematic elements)

Running time: 1:53

How to watch: Premieres in theaters June 28

Michael Phillips is a Tribune critic.