Martha Plimpton never thought she was cool. Not when she fought off villains in The Goonies, or wore plaid and dirty Converse before Kurt Cobain did, or went to the Oscars with an inch of hair and River Phoenix on her arm. But she’s grateful if anyone ever got that impression. “It makes me happy to know that all those years I thought I looked like a frump, something was working,” she jokes in her New York drawl, while chewing on a roll-up she won’t light for a while. She’s Lauren Bacall and Humphrey Bogart: her jagged glamour, his mouth.

It’s an overcast day in October, and Plimpton is reclining in a glitter bomb jumpsuit at one end of a hotel conference room. Her hair is cut into red waves, with a slick of blonde straight up the middle. It’s for a role, says the 51-year-old, who thinks of herself as a “hair actor”. She says she gets lost if it’s not quite right. For The Goonies’ no-nonsense Stef, it was cut into a shaggy bob, the kind made for adolescents who like to roll their eyes. As the rebellious teen dating Keanu Reeves in Ron Howard’s Parenthood, her hair was as long, blonde and pliable as a doll’s. For her new film Mass, she’s a grieving mother with a haircut reflective of her pain. It’s functional, efficient, fine. Bigger things are on her mind.

If awards season wasn’t so distracted by middling celebrity biopics, Mass would have been – and absolutely should have been – a major Oscar player. It stars Plimpton, Jason Isaacs, Ann Dowd and Reed Birney as two sets of parents (Gail and Jay and Linda and Richard, respectively) linked by a high school shooting. They’ve arranged to meet for the first time in the basement of a church and at the encouragement of a therapist. As the couples share uncomfortable small talk and simmer with guilt and fury, the context becomes clear: Linda and Richard’s son was responsible for the shooting, and Gail and Jay’s son was caught up in it. Both boys are now dead. It is a wrenching, difficult but ultimately life-affirming drama, one that dares to rest in a deeply uncomfortable space.

Plimpton found the script – by actor and first-time filmmaker Fran Kranz – impossible to put down. Kranz had become a father shortly before the Stoneman Douglas High School shooting in 2018. The Parkland, Florida massacre left 17 people dead and should have been, but depressingly wasn’t, a bellwether on gun control. Kranz asked himself whether he, as a parent, could ever forgive the parents of the person responsible. “He was overcome with questions,” Plimpton explains. “And even though we live in this horrible cultural place where we seem to always have these f***ing shootings, he didn’t set out to write a polemic. He wrote a story asking how we move forward and how we get out of this. And it is possible. It can be done.”

All four actors are at the peak of their superpowers, but I kept fixed on Plimpton. Gail burns with anger and quiet repulsion. She seems to despise her natural empathy at first, as if it makes her weak or disrespectful to the son she lost, but then it releases like a tidal wave. I seemed to follow Gail’s lead. At firstI villainised Richard and Linda, judging the showiness of their grief, but then later on I felt terrible about it. Plimpton understands that impulse.

“In our world right now, we don’t see everyone’s basic humanity,” she says. “The film demonstrates that we can stop having the Fox News or CNN version of the conversation. It’s done. We’ve covered that. What’s not been covered is: who are you? Who am I?” It’s easier said than done though, surely? She holds her hands up. “Hey, man, it’s difficult for me, too. We’re only human, and these things are designed to gin us up. That’s the reason these TV networks exist – they’re designed to polarise us. And if we’re polarised and divided, we’re much more easy to manipulate.”

That media ethos is one of the reasons she lives in London for much of the year now, even if she’s quick to clarify that she knows the UK isn’t much better politically. But, as an outsider, she can at least be distracted by very British pleasures: the less “self-serious” theatre scene; the shopkeeper on the corner who calls her “love” when she buys something; “getting a roast dinner at three o’clock on Sunday afternoon”.

In person, Plimpton shares the same forthright strength as her best characters, mixed with that underlying vulnerability. On screen, it’s kinetic: an enviable sense of total self-belief and beyond-her-years wisdom. The daughter of actors Shelley Plimpton and Keith Carradine, she found work as a child model and then an actor in the early Eighties, often playing smart alecs and tomboys. Art tended to imitate life. Plimpton’s life-long friend Ethan Hawke once remarked that she’s incapable of “buying any bulls***”. “I guess it’s the New Yorker in me,” she says. “I grew up in a city where you could walk and ride the bus and the subway with all kinds of people. It makes you less self-focused and more pragmatic.” Could she be intimidating as a teenager? “Only to misogynists. If people were ever intimidated by me, it was because of something they needed to work on in themselves.”

In 20 years I’ll probably read this interview and go, ‘F***, I sound like an asshole!’

A lot of those people did hold power back then, though. In early interviews, Plimpton would always vent about the state of the film industry. “I’m in a period of being extremely discouraged with the business and the lack of interesting roles for young women,” she said in 1988. “It’s completely chauvinistic.” When I remind Plimpton of this quote, she erupts in a cackle. “Jesus, I’ve been saying that for 40 years? F***ing hell, get a new line, lady!” When I tell her she was just 17 when she said that, she seems relieved. “OK, so not quite 40 years, thank God. But still.”

She says she agrees with the sentiments of her younger self, but doesn’t remember that era wearing her down. “When you’re a teenager or in your twenties, you think you know everything so you become a little bit of a dick,” she jokes. “I wasn’t the ingenue back then and it might have pissed me off – but then I grew up.” If anything, she finds the Martha Plimpton of 1988 a lot more frustrating than that era’s film industry. “You should definitely not ask anyone famous and under the age of 35 their opinion on f***ing anything. I read those interviews and I’m just like, ‘God, Martha – you sound like such a gas bag!’” She turns her eyes on me. “It doesn’t change, though. In 20 years I’ll probably read this interview and go, ‘F***, I sound like an asshole!’”

Hollywood was a long time ago. It was before she’d more or less quit filmmaking for Chicago’s famed Steppenwolf Theatre Company in the early Nineties, before her three Tony Award nominations and then her resurgence on television. She’s had a decade-plus run of playing flamboyant supporting characters in great TV: The Good Wife, for which she won an Emmy; the ramshackle family sitcom Raising Hope; Lena Dunham’s short-lived teen drama Generation. But everyone still wants to talk about what she was working on and who she was dating when she was a teenager.

She makes it a rule not to talk about Phoenix, and publicly hasn’t since a year after his death of a drug overdose in 1993 – they’d dated from 1986 to 1989. But that same caution also seems to extend to the films they made together: the paranoid thriller The Mosquito Coast, and the gorgeous Running on Empty, starring Phoenix as the child of counter-culture fugitives, and Plimpton as the girl with whom he wants to run away. She shares positive if oblique memories of that time. “I made some really fun movies with some really cool people who were really lovely and really taught me a lot,” she says. She grins, and says she needs a cigarette. Strutting across the room, she cracks open a window, wedges herself in its alcove, and lights up.

I ask what happens when you’re an impressionable 19-year-old still figuring out your place in the world, and then Jane Fonda plays your mother in a film – as she did in 1990’s Stanley & Iris. “It helps,” Plimpton deadpans. “I really, really love her. I love that she’s honest and that she’s trying, and that she hasn’t given up on her ideals about the kind of world she wants us all to live in. One of equality and justice and fairness.” Fonda and Plimpton share similarities – they’re women who’ve always balanced acting with activism, who’ve never drawn a line under their political education. For Plimpton, her primary cause is reproductive health.

“I care about the fact that my rights as a woman aren’t respected,” she explains, her voice crisp and measured. “And that women all over the world, but particularly poor women and women of colour, are even less respected. It makes me angry. I don’t want to live in a world where women aren’t welcome.” A few years ago, the Daily Mail expressed faux outrage over Plimpton’s presence at a pro-choice rally in Seattle, in which she joked that she had her “best abortion” in the city. She finds tip-toeing around the subject ludicrous.

“I don’t think abortion is weird or abnormal or something that ‘bad people’ do. Abortion is healthcare, and very basic and essential health care. And while it’s not just a woman’s issue – clearly, it’s also an issue for many trans men and many non-binary people – the roots of the stigma around it is misogyny. I’m sick of it. It deserves to be understood and respected and not framed in this very binary way: good and bad. And there’s no such thing to me as ‘pro-life’, you know? We’re all f***ing pro-life!”

A publicist arrives and asks us to wrap up. “Let me just take a breath and calm down,” Plimpton sighs. “It’s just very important to me, and I’ll be dedicated to it for as long as we have to do this bulls***.” She shakes her head, takes one last drag of her cigarette and tosses it out the window. Dusting the ash from her jumpsuit, she says her goodbyes and disappears in a sparkly flash down a corridor. In the Eighties, today and forever, she’s the kind of cool you want to be when you grow up.

Sky Original film ‘Mass’ is coming to Sky Cinema and cinemas nationwide on Thursday 20 January