Credits roll at the end of "Lady Bird," as they do at the end of every film, but this time they come as a shock.
The cascading names of cast and crew are unwilling acknowledgment that what we've been watching is not the most achingly real of real life but an exquisitely constructed, completely entertaining scripted drama.
Gerwig, whose credits include co-writing and starring in Noah Baumbach's fine "Frances Ha," has given us an elegantly empathetic portrait of a teenage girl's senior year in high school that will resonate even with those (no names) who feel they've seen enough teen-centric movies to last multiple lifetimes.
In this she's been helped beyond measure by Ronan, twice Oscar-nominated at age 23, most recently for the marvelous "Brooklyn," who's turned herself so completely into a Sacramento teenager circa 2002 it almost seems like witchcraft.
She did not, of course, do this alone. "Lady Bird," cast by Allison Jones, Heidi Griffiths and Jordan Thaler, has been especially shrewd in all its acting choices.
These include two of the hottest young actors around, Lucas Hedges and Timothée Chalamet as the boys in Lady Bird's life, a delightful Lois Smith as the head nun in charge of her parochial school and Tracy Letts as her understanding father, Larry. But the most crucial choice of all was Laurie Metcalf as Lady Bird's mother, Marion.
For though "Lady Bird" deals with the whole panoply of senior year experiences, the heart of things is that particular parent-child dynamic. As Gerwig herself has succinctly said, "the mother-daughter relationship is the love story of the film."
Best known for her stage and television work (a Tony this year, three Emmys for "Roseanne") Metcalf's extraordinary performance underlines the close but combative nature of this fraught connection between people whose intense feelings turn on a dime from pleasure to pain, who can count most of all on always driving each other crazy.
"Lady Bird" begins with an image — mother and daughter facing each other on a bed, more or less mirror images — and a pithy sequence that underline the fractious duality at the heart of things.
Together in a car, Lady Bird and Marion bond and bicker at a dizzying pace. After sharing tears over an audio version of "The Grapes of Wrath," Marion says "let's just sit with what we've heard," with Lady Bird coming back with "Are you serious?"
When the talk turns to Lady Bird's college plans after her senior year at Immaculate Heart in Sacramento ("the Midwest of California"), the divide widens precipitously.
Lady Bird insists she wants an East Coast liberal arts school, explaining she "wants to go where culture is." Her mother thinks that's unrealistic and bitingly ripostes, "How did I raise such a snob?" Lady Bird's rejoinder, and it is a memorable one, has to be seen to be believed.
Though she insists Lady Bird is her given name ("I gave it to myself"), Lady Bird, as it turns out, is not even this teenager's birth name, which is Christine.
But this blithe desire to reinvent herself is par for the course for the willful, oblivious, funny and achingly sincere young woman Ronan animates with a delicious performance that is natural, unaffected and absolutely on target.
While Gerwig, who is herself from Sacramento, has insisted that none of the events in the film literally happened to her, they cut so close to the bone that one can't help but wonder.
These include chaperones at a school dance insisting on leaving "six inches for the holy spirit," a college counselor bursting out laughing when Lady Bird mentions Yale, and a football coach who directs theater as if he were diagramming plays.
Also on the scene are the best friend, irrepressible Julie (Beanie Feldstein), classic cool girl Jenna (Odeya Rush), the irritating older brother Miguel (Jordan Rodrigues) and his live-in girlfriend Shelly (Marielle Scott), who tells a disbelieving Lady Bird that "your mother has a big heart."
And not to forget those boys, classic types each one. Straight-arrow Danny ("Manchester By the Sea's" Hedges) is the one even a mother could love, while Chalamet, the breakout star of the forthcoming "Call Me By Your Name," gets self-involved bad boy Kyle just right.
Convinced that "we're done with the learning portion of high school," Lady Bird is taken aback to realize, slowly and reluctantly, that life still has lessons to teach that are harder to learn and not easily disregarded. Not exactly pure pleasure for her, but we wouldn't miss it for the world.
Rating: R, for language, sexual content, brief graphic nudity and teen partying
Running time: 1 hour, 33 minutes
Playing: ArcLight, Hollywood, Landmark, West Los Angeles