“Grandma” opens Friday at Sundance Cinemas. R, 1:22, three stars out of four.

“Time passes. That’s for sure.” The epigram from poet Eileen Myles opens writer-director Paul Weitz’s comic drama “Grandma,” and the film doesn’t get much more profound than that in exploring its themes of aging and regret. But Weitz (“About A Boy”) isn’t aiming for sweeping profundity, instead making a character study that’s small, sour and sweet — like one of those hard candies your grandma had in a bowl by the front door.

Elle Reid (Lily Tomlin) had no such candies by her front door, and if she did she’d more likely use them to throw at people who rang the doorbell. An acerbic, tart-tongued poet, Elle uses her considerable skill with language to find just the right words to wound others. As the film opens, she’s breaking up with her girlfriend of four months (Judy Greer), dismissing her as a “footnote” in her life. But as we later see Elle sobbing in the shower, we sense she may not be as tough as she talks.

And then appears Sage (Julia Garner), Elle’s teenage granddaughter. Sage is pregnant by her good-for-nothing boyfriend and wants to get an abortion, but doesn’t have the $600. Elle is also broke, but reluctantly agrees to find her the money somehow and take her to the clinic, without Sage’s overbearing mother Judy (Marcia Gay Harden) finding out.

So most of “Grandma” is a picaresque journey of sorts, as grandmother and granddaughter wander Los Angeles looking for the money. The trailer for “Grandma” makes the film look like it’ll be all about the sassy Elle dressing down all the jerks she comes in contact with — Watch her give the fuddy-duddy barista a tongue lashing! Watch her knee Sage’s stoner boyfriend in the balls! That’s all there, and pretty funny thanks to the magnificent comic talents of Tomlin.

But “Grandma” goes deeper, as Elle and Sage’s quest ends up digging up old memories for Elle, returning her to the scenes of some emotional crimes she’d rather forget. We learn that she’s grieving the death of her longtime lover Violet, and get the sense that Violet was the half of the couple that everybody liked best. There’s a combustible scene with the late Elizabeth Pena as an old friend, and a tender one with LaVerne Cox as a trans tattoo artist.

The scene that “Grandma” hinges on, one of the best of 2015, is Elle’s confrontation with her first husband Carl, who she abandoned for Violet. Karl is played by Sam Elliott, and when we first see him seems the picture of contented old age — big house, lots of grandchildren, even a flat belly.

As their conversation goes from how-you-been chitchat to something deeper, the old anger and sadness in Carl starts to erupt, against his will. It’s a brief but powerfully raw performance by Elliott that puts the lie to the idea of old age as a mellowing time.

After that moment, “Grandma” seems to coast a little, and the film isn’t helped much by the addition of Harden, a fine actress saddled with a too-broad characterization of a high-powered executive (a treadmill desk!) that she has to overcome. But as the film comes to its gentle, feel-better (if not feel-good) conclusion, we’re always in good hands with Tomlin.

With her discerning eyes and mirthless smile, she makes Elle a force to be reckoned with. Even if, nearly alone at the end of her life, the sum of her hard choices, she’s now the force she has to reckon with.