Many people don’t want to see Ann Dowd’s new movie. Even the most positive of its reviews from Sundance called it “excruciating”, “exhausting” and “tortuous”; an endurance test some will not be willing to endure. Including Dowd herself, who has yet to watch it.

“We’ve talked about it a lot, the cast, and we have different points of view,” Dowd says to me over coffee in Chelsea, New York, conscious that Mass is a tough sell. “When people ask me, I say this film has tremendous hope and that it has to do with healing and forgiveness. I don’t give the specifics.”

The specifics are such: two sets of parents meet six years after a school shooting. Both lost their sons that day – one was a victim and the other the killer. The conversation that follows is contained in just one room for the duration, uncomfortable and inescapable. Dowd plays the mother of the shooter.

Dowd, now 65, has lately found something of a niche in mainstream psycho-horror, as the fearsome Aunt Lydia in The Handmaid’s Tale, the malevolent Patti on The Leftovers and the downright frightening Joan in Hereditary. But even she was trepidatious about this one. “I wondered, can I go to that level of grief and stay there for the time needed to tell the story respectfully and honestly?” She can, it turns out: four years after her Emmy for The Handmaid’s Tale, Oscar buzz now surrounds her.

I’d met Dowd briefly a few weeks ago; today she acts as if we’ve known each other for years. She brushes her dog Chance’s hair off my jeans, calls me “honey” or “babe”, offers help for my mother’s upcoming visit (“You have everything? Like pots, pans?”), moons over my name (“the most beautiful … it says peace to me”) and strokes my hand as we speak (“I keep going for your thumb, I’m sorry. It’s the mother in me”). It’s not the Dowd we’re used to seeing on screen.

“Fans are lovely,” she beams. “They just think that I’m definitely going to be the meanest person they’ve ever met.” A Handmaid’s Tale fan once ran away from her in public. She’s unsure, too, about the discrepancy between her personality and her current pigeonhole. “I don’t know why it’s a good fit. I’ve always been drawn to loners, I’ll tell you that.”

Dowd worked consistently but quietly throughout the late 1980s and into the following decades. She was Tom Hanks’ sister in Philadelphia; Natalie Portman’s mum in Garden State, multiple characters in the Law & Order universe. But leading roles eluded her. “I used to weep after every audition I didn’t get,” she says. “I lost a ton of weight, was the thinnest I ever was. I was living in LA at the time and I thought I’m gonna get a billion roles. I’m not gorgeous or anything but my face is fine. Didn’t get a thing. Weep! Then finally in the middle of a weep one day, I just stopped dead and I said: you are choosing this reaction, choose another reaction. That’s the last time I wept over losing a role. That was 22 years ago.”

It took 2012’s Sundance hit Compliance, a grim drama about a horrifying escalation of abuse at a fast-food restaurant, to suddenly move her from the sidelines and into the centre of the frame. Dowd was a week away from her 56th birthday when it premiered. These decades of experience, along with a background in training actors, has leant her a robust scepticism about pontification in her trade.

“Acting is not suffering, and I swear by that,” she says. “At the end of the day we go home and we don’t carry with us the consequences of the story. That’s the only reason you can do it. When I’ve been on set with young actors who are very method, it worries me terribly. And I want to say to them, Sweetheart, come on now, you don’t need to have a nervous breakdown just because your character does, it’s about the imagination, Honey, that’s your gift.” During Mass, she remembers “howling with laughter” between takes.

But lately, she says, a creeping gloom has entered the picture. “I don’t know if it’s the pandemic or not but I’m experiencing anxiety that I have not had before and talking about this film is much harder than filming it. As soon as we started talking about it, I would just start weeping because I wasn’t accustomed to going back to that story as an outsider. But now I realise I have anxiety around things I didn’t even know I did.”

Our allotted time is up, but the conversation continues as quasi-therapy, sharing dovetailing stories of family tragedy and hardship. Dowd listens as much as she talks; and when she talks, it’s largely off the record. A big theme turns out to be men unable to express their despair, and the dangers that can come with that.

“This is gonna sound really horrible and you can tell me to fuck off,” she says, “but I think because women are the ones who bear children, we can handle it. But not only are men to me more vulnerable but to tell them to then shut that down compounds it. It’s like: no, what do you think they’re supposed to do?”

Dowd has three children, the eldest of whom is on the autism spectrum and the youngest was adopted at six. Each, she says, brings with them a different challenge. But she’s pleased they seem able to share with her when they’re feeling low – something her character’s son in Mass does not. “She lost her son and he was responsible for the deaths of others and the destruction of families,” she says. “But the one that dawned on me latest was that her son was in that level of despair and she did not know it,” she says. “He was suffering.”

But while in person Dowd might major on maternal warmth, flickers of something steelier do come through. She’s acute recounting a story about a woman who was rude to her in Macy’s. And she doesn’t shy from discussing a brusque soap director she clashed with early in her career.

One guest star on The Handmaid’s Tale – one with a “difficult reputation” also comes in for some dressing down. “She drove everybody crazy,” says Dowd. “I was like: Babe, no need. That to me is like a psychological issue, where you think: is there an insecurity, sweet girl? Because you’re established, we’re all kind of nice around here, nobody’s important, so what’s the problem?”

And then I witness Dowd’s fearlessness in person, when we spot an aggressive guy (with a dog with a similar temperament) arguing with a young woman in the corner of the coffee shop. Dowd stops dead, like a superhero suddenly called into action. “Sweetheart, are you OK?” she calls over. “You’re doing just fine?”

The man barks back, telling her to “shut your nosey old trap”. I start to get involved but Dowd reaches for my hand and backs me down: “No worries, no worries. I just want them safe.”

It’s the sort of everyday incident that can nonetheless leave you slightly shaken. But Dowd seems entirely unmoved: all resolve and watchfulness. “Go home, Sir,” she says under her breath later on, when the man’s dog begins yapping, with the air of someone conjuring a spell. It’s a brief snapshot of the resilience that’s propelled her through her dogged rise and will undoubtedly help her power through what she calls her latest “challenging” bout of anxiety.

“You have to square off with it,” she says. “You can’t shove it away. You have to settle with it and think you’re gonna be all right.” And she holds my hand.

• Mass is released in cinemas and on Sky Cinema in the UK on 20 January 2022.