From the moment we meet Marnie Minervini, the smothering mother in Lorene Scafaria’s “The Meddler,” it’s clear she can’t be a total fiction. Everything about her has the ring of truth, from her nattering voice mails to her habit of beginning most sentences with “You should.” Marnie is based on Scafaria’s own mother, of course, and what comes through in “The Meddler” is the filmmaker’s deep love for this wonderful, maddening woman.

It helps that Marnie is played by Susan Sarandon, sparkling as ever as a widow who relocates from New Jersey to Los Angeles to “help” her daughter, Lori (Rose Byrne), a struggling television writer. Lori, too, is pining for a lost love, Jacob (Jason Ritter). Sarandon and Byrne are terrific as two women who are grieving and doing a fairly bad job of it. Lori exhausts herself with work while Marnie intrudes ever further into her daughter’s life. Marnie casually delivers the movie’s funniest and most horrifying line: “Oh, and remind me to tell you what your therapist said.”

“The Meddler” may be based on familiar archetypes, and it may not have much of a plot — a turning point comes when Lori flies back East for work, leaving her mother adrift — but Scafaria’s fine writing and carefully chosen cast carry the day. What makes the film such a winner are the little moments, as when Marnie impulsively pays for the wedding of Lori’s gay friend Jillian (Cecily Strong), treats a helpful young Apple employee (Jerrod Carmichael) to ice cream and accidentally wrecks a film shoot only to become a beloved member of the cast.

That little meta-twist is how Marnie meets Zipper (J.K. Simmons), a retired cop who begins courting her with gentlemanly patience. Watching Simmons and Sarandon together — two supremely confident actors with lived-in charm — is one of this movie’s great rewards.

If “The Meddler” has an off-note, it’s Marnie’s cartoonish Italian-American in-laws, who literally eat pasta to the strains of “Santa Lucia.” Those brief scenes mar the movie only slightly. Overall, “The Meddler” is a light and lovely gem.