Young people are the only ones who ever talk about growing old gracefully. For those actually in the thick of it, the romance of that notion burns off pretty quickly, and wrinkles and creaky joints are the least of it: Growing old, gracefully or otherwise, means becoming the person you were always meant to be, only more so. After days, months and years of gradual transformation, you wake up one day to find that you're 1,000 percent you. Your good qualities have entwined so fixedly with the bad that it's hard to distinguish which are which. By the time you feel wholly comfortable in your own skin, everyone around you may find you unbearable.

Lily Tomlin's performance in writer-director Paul Weitz's Grandma doesn't just hint at that idea — it lives in it. The movie gets off to a shaky start, working too hard to establish the unrepentant prickliness of Tomlin's character, widowed poet Elle. But it gradually settles and deepens into something nuanced and moving, a character study that's not so much about aging, specifically, as it is about the great and awful process of getting to know yourself. As Elle finds out, even when you think you know everything, there's always more to learn.

Grandma opens with a breakup, a rather vicious one: The 70-ish Elle is calling it quits with a younger woman we quickly ascertain is her girlfriend — it's the sort of breakup where, believing you know where a relationship is headed, you drive the knife in farther and deeper than you need to, preemptively wounding your partner more than he or she could ever hurt you. The girlfriend, Olivia (Judy Greer), stands dumbstruck. As Elle's tirade escalates, we learn that the two have been seeing each other for only four months, but that Elle has always known the age difference would eventually become a problem. We learn that although she's been writing poetry lately, she's been lazy about getting it published. Writing poetry is all about paring language to the essential minimum, and we learn just how good Elle is at cutting people down, too. After making reference to a previous partner, one she'd been with for nearly 40 years, Elle tells Olivia, “You're a footnote.”

Who says a thing like that? Usually a person who doesn't mean it, but that never lessens the sting. Once Olivia, understandably hurt and angry, takes her leave, we learn that Elle isn't as brittle and heartless as she thinks she is, a truth that Tomlin imparts to us resolutely but with her back to the camera, as if she were guarding Elle's privacy.

This is just the beginning of the tart, subterranean grandeur Tomlin, who has always been a marvelous actress but who hasn't had a leading role in nearly 30 years, brings to Grandma. This is her second film with Weitz — she also appeared in his last movie, 2013's Admission — and though the director has said that he'd had the basic idea for Grandma for years, it wasn't until he met Tomlin that he knew exactly how to write the character. Tomlin fills out the role like a tree spreading its branches and roots, though she brings a superb lightness to it, too: Elle's acidity often has a comic kick — for her, wisecracks aren't just a defense mechanism but a means of surviving the worst.

And she needs those wisecracks, perhaps now more than ever: Her granddaughter, Sage (Julia Garner), shows up on her doorstep, announcing that she's pregnant and needs $600 for an abortion. Sage is so ethereal-looking you almost can't quite believe she could conceive a human child — with her halo of pale-blond curls, she's like an Arthur Rackham fairy. But she's all too human, and Elle, after briefly berating Sage for her carelessness, agrees to help, even though, having recently cut up all her credit cards, she's low on funds herself. The two pile into Elle's car, a bumptiously elegant 1955 Dodge Royal (Tomlin's own, bought in 1975), in search of the money Sage needs to solve her problem.

During this road trip, the thorny, multi-dead-end map of family resentments is laid out. Sage doesn't dare tell her distracted businesswoman mother, Judy (played, with both sharpness and subtlety, by Marcia Gay Harden), about the pregnancy. Elle isn't speaking to Judy, either — the two have had a falling-out. What's more, all three women are still in mourning for Violet, Elle's longtime partner, Judy's mom and Sage's other grandmother: She died not so long ago, after suffering through an illness.

Grandma is a multigenerational story in which men are side players, though they're not completely negligible. In one of the movie's finest and most piercing scenes, Elle and Sage seek out one of Elle's old friends, who, it turns out, was at one time something more. Karl (Sam Elliott) is happy enough to see Elle, but he's puzzled too. What follows is part tender reconciliation, part brutal showdown. It's also a point of reckoning for Elle, who has a seemingly endless ability to inflict damage on other people. (Elliott was recently seen in Brett Haley's intimately magnificent comedy-drama I'll See You in My Dreams, and he's just as terrific here.)

Elle seems not to care that she has caused pain — and yet she betrays, in the smallest of ways, that she does care. In one of the movie's slyest moments, Elle, a woman whose beauty is of the no-makeup and uncombed-hair variety, adds a swipe of lipstick just before she and Sage head off to see Karl — there is no woman who doesn't occasionally feel the need for some sexual armor. But Elle's calculation is all right there on the surface, not just on her lips but also in her quips and her barbs. Her tenderness runs much deeper, as she shows in a late scene where she reconnects with Olivia, perhaps showing her erstwhile lover more kindness in 10 minutes than she'd shown during the course of the duo's brief relationship.

Temperamentally, Tomlin's character in Grandma is nothing like the one she played so long ago in Robert Altman's Nashville, a dutiful, married mother of two who succumbs to the seductive charms of Keith Carradine's singer-songwriter Casanova. But watching Tomlin here, as a woman who is 1,000 percent herself and could perhaps use a little dilution, I kept thinking of that scene in Nashville — possibly its most beautiful — where Carradine sings to Tomlin from the stage of a packed club. She listens, her face immobile but as filled with feeling as a cupful of tears. She knows that even if he has dozens or hundreds of lovers, this song is only for her. The song is everything, and not just for the moment — it's something to be carried forward for the rest of life, even after the lover is long gone. Tomlin packed a lifetime of future feeling into those few moments of Nashville. In Grandma she shows us the aftermath of the song. The short version: Life went on. It was terrible. It was wonderful.

GRANDMA | Written and directed by Paul Weitz | Sony Pictures Classics | Landmark, ArcLight Hollywood

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