Of all the movie protagonists you might have seen this year, none is quite like Marianne Jean-Baptiste’s Pansy in Mike Leigh’s “Hard Truths.”

Pansy, a middle-aged woman in contemporary London, is foul-tempered from beginning to end. She spends her days in evident pain that she unleashes upon all those around her, including her husband Curtley (David Webber), her 20-something son Moses (Tuwaine Barrett) and most anyone else she encounters. Her venom might fall on a supermarket cashier or a furniture-shopping couple who dare to put their feet up on an ottoman. Heaven help the man who wants her parking spot.

For everyone, Pansy is a test. She tests the patience and empathy of her family, just as she does the viewer. She’s not an antihero, she’s anti-everything.

“The world is full of Pansys. People live with other people’s conditions,” Jean-Baptiste says. “Often I’ve met people who have just been enraged, because you didn’t see them in the car park pulling into the space. You go: It can’t just be about me. How did you get that angry about something so stupid? You don’t know what they’re going through or how they got there.”

“Hard Truths,” which will open nationwide in theaters Jan. 10, never supplies any answers. Instead, it unfolds as a cantankerous character study, led by Jean-Baptiste’s compellingly prickly Pansy.

The performance has earned Jean-Baptiste her best reviews since her last film with Leigh: “Secrets & Lies,” nearly 30 years ago. For that film, Jean-Baptiste became the first Black British actress nominated for an Academy Award. Her performance in “Hard Truths” has been just as celebrated, earning best actress from both the New York Film Critics Circle and the Los Angeles Film Critics Association. Three decades later, Jean-Baptiste could be heading back to the Oscars.

“You sit down with Marianne a hundred years later from ‘Secrets and Lies,’ in which she played a very intelligent young woman, and Marianne has now moved on in life,” Leigh says. “We love each other because she’s very, very funny. So sitting down with her ability to be real and profound but also grotesque, that, alone, points me in the direction of possibility.”

Making a film with Leigh, the 81-year-old humanist master of “Naked,” “Vera Drake” and “Mr. Turner,” isn’t a typical process. There’s no script to begin with, just an invitation.

“It was the usual: Let’s do a film,” says Jean-Baptiste. “Don’t know anything, but let’s do it.”

Leigh pulls his characters and storyline out of months of rehearsal. In the case of “Hard Truths,” they rehearsed for three months — somewhat short for Leigh (it was six months for “Secrets & Lies”), but extremely lengthy for today’s movie industry.

“Normally you get on set it’s like, ‘Uh, this is Ralph. He’ll be playing your husband. You’ll stand over there,’” says Jean-Baptiste.

In Leigh’s manner of rehearsal, they begin with a character’s first memory, and then flesh out their life all the way up until the time period of the film. But there are parallel histories for other characters that require constantly going back through and recontextualizing. Meanwhile, costume designers and production designers await clarity on what kind of clothes and homes they should craft.

“All the decisions about the character that you can make, the actor makes them,” says Jean-Baptiste. “Any of the decisions that God makes for somebody’s life, he makes them. So it’s like: She wants this job, so she applies. A letter arrives in the mail: Unfortunately you did not get it.”

Jean-Baptiste was a recent drama school graduate, classically trained and oriented toward theater, when she co-starred in 1996’s “Secrets & Lies.” It was her breakthrough. A few years after that film, she moved to Los Angeles, where she’s been since, acting in a wide variety of projects including the TV series “Without a Trace,” “Blindspot” and “Homecoming.” Asked if her collaboration with Leigh had changed from “Secrets & Lies,” Jean-Baptiste said much was the same.

“Obviously we’re a lot older,” she says, smiling. “I think we just slipped right back into it. He was gentler.”

Part of what makes Jean-Baptiste’s performance as Pansy so uncanny is how unlike Pansy she is. Jean-Baptiste is charismatic, laughs frequently and enjoys throwing herself into uncertain circumstances (like “Hard Truths”). Asked if she has anything in common with Pansy, she replies, with a laugh, “I hope not.”

“I have a sense of humor she doesn’t, although she’s really funny,” Jean-Baptiste says. “I think I recognize that part of myself to the extent that I’m like: That’s not how I want to live. That’s not how I want to be.”

But in Pansy, Jean-Baptiste recognized people she knows, and the kind of character that seldom makes it onto movie screens. “A difficult Black woman, you don’t see that,” she says. “I don’t think I ever have.”

In “Hard Truths,” the root of Pansy's depression is uncertain, but a sense of festering wounds from the past is palpable. When she speaks to a doctor, she sums it up: “The heart of the matter is me head.” Later, when asked why she can’t enjoy life, she replies, “I don’t know.” Jean-Baptiste, in mapping out Pansy’s history, has some theories about what’s made her this way.

“She had a number of issues that went unaddressed and found coping mechanisms to get through life,” says Jean-Baptiste. “I think a lot of people are undiagnosed with things and just make do. Maybe she’s one of those.”

“The fear,” she adds. “It was the fear that I focused on the most. She attacks before anyone can attack her, and she thinks everyone is attacking her.”

But Pansy’s specific diagnosis isn’t the point of “Hard Truths.” It's much more about how her family and the outside world react to her. She might be pushing everyone away, but it's clear she's in urgent need of something.

“I want so desperately for someone to help Pansy,” Jean-Baptiste says. “I think it would be very convenient to go, ‘She’s got this mental illness or that’s what’s wrong with her.’ But what’s more interesting is that we don’t actually know and she’s just in pain.”

“Hard Truths” ultimately ends in a kind of cliffhanger, with Pansy and her family locked in stasis. If Pansy tests the boundaries of empathy an audience might feel for a character, it's a moment of truth: Does Pansy go to her husband or refuse to budge? Jean-Baptiste wants to believe in her.

“I’d like to think that she goes, I do, because I like her,” Jean-Baptiste says. “I like Pansy. I gotta look out for her.”