Fly away home: "Lady Bird"

In her screen career to date, the 34-year-old actress Greta Gerwig has shaped up as a very striking, apparently super-relatable presence: wearing a superficial sophistication, applied like morning-commute mascara, over a bedrock of scatty millennial disorganisation, she's suggested a new and more grounded variety of American sweetheart, caught with arms aflap, running in vain after a disappearing bus. More credulous observers, unable to separate the life from the art, might have been given cause to wonder whether such a figure would ever be able to keep it together long enough to write and direct a feature of her own; yet they should bear in mind that even the la-di-da Annie Hall went on to sustain a reasonably successful career behind the camera, and that Gerwig has been close to the director's chair for several years now, most recently in her collaborations with Noah Baumbach (Frances Ha, Mistress America) and before that as part of the close-knit role-swappers who made up the Noughties mumblecore movement. (She already has one co-directorial credit, working alongside Joe Swanberg on 2008's Nights and Weekends.)

What strikes us immediately about Lady Bird, Gerwig's first effort flying solo, is how well and how instinctively the woman behind the camera appears to know her characters and her material. Granted, the writer-director has re-entered the fray with a semi-autobiographical coming-of-age tale, a field within which countless neophytes have taken their first, cautious authorial steps, but the film's hyper-accelerated editing rhythms - which are novel, somehow very Gerwig, and take some getting used to - suggest a filmmaker working hard to process her memories and cram in the most vivid among them; these choices derive not from scattiness, rather strategy. In this, the most condensed 94-minute feature of the season, every cut provides its own form of closure on one rite-of-passage or another, while opening up the possibility that life might be going on elsewhere, beyond the ken of its rangy yet relatively clueless lead. A few months flash before our eyes, and the follies of a girl in late adolescence come to be reappraised from the perspective of an almost fully grown woman.

It should be noted that this isn't strictly Greta Expectations: Saoirse Ronan's heroine bears the given name Christine McPherson, although she's rechristened herself Lady Bird, a typically teenage act of rebellion that reflects her status as one about to fly the nest for college, but which also never stops seeming affected and silly. Yes, the luminous-pale Ronan bears a passing resemblance to the younger Gerwig, the character grows up in the humdrum Sacramento of her creator's youth, and yes, there are details and incidents that strike us as too specific not to have been lived through in some awkward or painful way: Christine's bathroom-basin dye job and clove cigarette habit, say, or the scene that finds her demanding her parents work out how much it cost to raise her, so that she can reimburse them in later life. Yet the emotions the film goes for and gets at are universal. Christine tangles with boys (Lucas Hedges as a sensitive fellow drama student; Timothée Chalamet as a brooding non-conformist who could only be a member of a band called L'Enfance Nue), changes best friends as it becomes plain one of her classmates is developing at a faster rate than the hesitant Julie (Beanie Feldstein), and comes to terms with shifts within her family unit.

Here, the film reveals its greatest economy: performers capable of sketching whole lives with a handful of words. Playwright Tracy Letts gives another of his skilful, subtly expansive supporting turns as Christine's father, a pill-popping sadsack deflated by the task of making ends meet. (If ever you wanted to see a man who needed rescuing from the burdens of patriarchy.) Yet the heart of the film is one of the most believable mother-daughter relationships in modern cinema. Christine's mom Marion, a health professional, is prone to domestic microaggressions that wind our wannabe free-spirit heroine up no end, but which the older Gerwig now has the wisdom to sense were the efforts of a houseproud lower-middle-class woman to keep up appearances and raise her offspring right. The excellent Laurie Metcalf does the character the honour of playing Marion as an actual woman with real, bruisable emotions rather than the fussing sitcom caricature she might have become, and the switch in perspective proves to be key. This blithe entertainment doubles as an overdue love letter to a parent whose sorry fate, during her daughter's formative years, was to be bawled out on an almost nightly basis - a way for Gerwig to repay her mom by making the effort to understand her. (Cinema bookers: bear in mind that March 11th is almost upon us.)

In form, Lady Bird bounds up to us resembling one of those familiarly sunny, easy-to-sell indie ventures that were ten-a-penny back in the 1990s, before the market got saturated, the credit got crunched and the mumblecorers were obliged to rebuild the movement from scratch. (For Harvey Weinstein and Miramax, swap in those altogether healthier souls over at A24.) Yet it's testament to Gerwig's originality of outlook that the film never once succumbs to the usual Sundance Lab formula (for one thing, I don't think you'll be able to see its final movement coming, and it's all the more touching for that), and instead sustains itself and its audience by a buoyant generosity of spirit - the same spirit that might well carry a young creative from one spot on the planet to another, from a suburban backwater to the red carpet of the Academy Awards. Late on in the film, Christine's Goth brother Miguel (Jordan Rodrigues) responds to his sister's rapid progress through the world with a scowled "Whatever you're up to, it won't end well." Lady Bird was what the slyly, studiously observant Gerwig was up to all along, and - against everybody's expectations, quite possibly even Gerwig's own, waiting in the rain for that next bus - it really has.

Lady Bird is now playing in selected cinemas, and expands nationwide on Friday.