2003 feels like a long time ago. But Samantha Morton remembers it well. She was supposed to star in the Terry Gilliam film The Brothers Grimm, and yet she didn’t. The reasons were not the kind openly discussed in the film business of the 00s. But Morton discussed them. She talked publicly about the fact Harvey Weinstein had ordered Gilliam not to cast her. His reasoning was alleged to have been that men would apparently not “want to fuck that”. Morton talked about Weinstein and the episode to journalists. She talked to her peers. She told a newspaper six other women later refused to star in the film.

“I just remember thinking, it’s a shame you behave this way. You’re older than me, you have this vast power. But if someone is bullying you, you have to do something, don’t you?” She pauses as if a thought had just struck her. “Maybe I was the first person to publicly answer him back.”

Morton is in Atlanta, Georgia. In any given conversation, her saucer eyes suggest a dozen different moods – pensiveness, outrage, hilarity. Now, she mostly looks surprised to be here. She arrived in the US last month to join the enduring zombie saga The Walking Dead, the job coming out of the blue, too good a chance to say no to, even if it meant relocating her family (she has three children, aged four to 18). Her time since has been consumed by the long days of high-end TV. The workload is its own bubble – she hasn’t had time to gauge the mood of the country off set. In a few days, there will be a red carpet season premiere in LA. “Chateau Marmont stuff,” she says.

I last saw Morton a year ago, in a car park outside Newport County football club. It was drizzling. She was in south Wales making a low-budget British film called Two for Joy, directed by the young photographer Tom Beard. The role was the kind it often feels only Morton can play, a single mother upended by depression. Shooting in a house nearby, she lay in bed, dazed and helpless every time the camera rolled. The second a break was announced, she bowled out with a big kid grin and wave. “I’m not method,” she says now. “I come off set and I’m Sam. If we’ve got the shot, I’m a happy bunny. What’s not to smile about?”

Two for Joy is impressive. But Morton is the ace – painfully alive, endlessly human. “Is the film good?” she asks. “Good. I thought it would be.” She hasn’t seen it? “Oh, I never watch films I’m in. It ruins them for me!” I ask why and she ponders the question. “Dunno.” A yelp of laughter. “It started off not liking how I looked. When I was little, it was a vanity thing. ‘Is that me? Is that what my lips look like? Ugh!’”

Morton and Billie Piper in Two for Joy Photograph: PR

These days, she says, she wouldn’t want to see what she was doing on camera. She doesn’t want to understand it. The cliche about actors is that they need constant reassurance. For Morton the drive feels different – the point is to work on her own terms, in her own way.

“I like being on the outskirts. When I was younger and the chance came along to be, for want of a better word, a movie star, I declined. So with Weinstein, although I knew he was a bully, I wasn’t aware of the rest of it. I kept that world of his away from me.” She speaks slower now, double-checking every word. “I don’t know if people have noticed, but I have been very quiet about Harvey Weinstein, and very quiet about Me Too. And it’s not because I don’t care. It’s because I’ve cared about this since before I could speak. It’s very close to home.”

Morton grew up in Nottingham in a series of care homes and with various foster parents. In 2009, her experiences shaped the one film she has so far made as a director, The Unloved. When it was released, she gave an interview to the Guardian disclosing that she had been sexually abused as a child. “It’s very simple. Sexual abuse is not something to ever be dismissed. If somebody says another person has done something to them, we listen. It is a duty.”

In Newport, she was shooting Two for Joy in a cramped council house on the Alway estate. “I grew up in houses like that,” she says. Her history still informs her professional choices. Even on the blockbusters she takes, the kind that pay the bills – The Walking Dead or JK Rowling’s Fantastic Beasts – she tries to apply the same logic as with a film like Two for Joy. “I just think, can I do anything with this? Are the film-makers nice people? Not ‘nice people’, but people I can be vulnerable with.”

For all her love of acting, she often talks beyond it, out into the real world. When she mentions her research before Two for Joy – demanding Beard specify the medication her character was taking, hitting Google to pinpoint the side-effects – I ask if she thinks attitudes to mental health have improved in Britain.

“There is a dialogue about mental health now that there wasn’t for many years. But our government is completely devoid of understanding about what triggers mental health problems, which is poverty, debt, a lack of respect and dignity. Suicide rates among young men through the roof, single mothers demonised, young people’s mental health provision underfunded. It is disgusting. It is horrific.”

For Morton, the subject cannot end there, bound up with wider problems, more affronts. In a furious monologue, she touches on Grenfell, school budgets, gambling adverts paraded before families watching the World Cup (“ITV, have you no shame?”), the ransacking of the NHS, the “clueless” treatment of Northern Ireland in Brexit negotiations. “And it all comes from the same root, which is a government I am mortified and embarrassed to see running our country. Has Theresa May got one scrap of genuine concern for what her policies do to ordinary people? No. They just do not care. And we feel so superior because we didn’t elect Trump! We’re hypocrites! It makes me judder.” She laughs, fleetingly.

Did she find it a relief to get out of the Britain of 2018? “Yeah. Yes, actually. It was.”

Morton is only 41 but you can mistake her for having been around for ever. She started acting before she was in her teens, starter jobs in Soldier Solider and Cracker coming while she was living in care. She was still in her early 20s when her signature roles arrived – Woody Allen’s Sweet and Lowdown, Steven Spielberg’s Minority Report, the title role in her friend Lynne Ramsay’s Morvern Callar. In 2003, there was an Oscar nomination for her turn in the New York immigration drama In America.

Yet Hollywood and Britain both proved to have limited space for her talent. With her otherworldly presence, she was wrong for the underwritten wives that were still then the norm for female roles. And there was her “difficult” reputation. “Now there is more respect on sets for what I’ve done, but in my 20s, certain kinds of people said I was a nightmare. That I’d create problems.” Such as? “I was vocal about conditions on set. I’m political. I’m outspoken about the way child actors are treated. About the hours people have to work. I don’t accept grotty behaviour. I don’t accept the misuse of power. I’m a strong woman and they don’t like it. They’ve never liked it.”

Fifteen years ago, talking back to Harvey Weinstein would not have helped. Fate brought other troubles. In 2006, she suffered a stroke after an injury sustained during a house renovation. The causes were myriad, the result that now, on the big screen, perhaps the most gifted film actor in the country juggles supporting parts in Fantastic Beasts with films so small they fall through the cracks. “A lot of films I do now, people never even know exist,” she says.

Yet she is tickled by the idea there will be untold viewers seeing her for the first time in The Walking Dead without ever knowing she was once Morvern Callar. “I like that! I enjoy being a secret. Like the first time you see Dennis Hopper and then you realise he had this mindblowing career.” There are no regrets. “I made all my decisions with the best information I had available, so regret doesn’t come into the equation. Back in the 50s, how many amazing women were caught in the studio system? I am so lucky. I’m a working actor and I’m not owned by anybody.”

In the mayhem of 2018, Morton is getting by. She is due back in Britain this year to film her last series of the TV show Harlots. Beyond that, she says, all bets are off. “England frightens me more now than it ever did under Thatcher. I just tell myself stay strong, Sam.” She smiles a huge cartoonish smile. “And don’t let the bastards grind you down.”

Two for Joy is on release in the UK now.

• This article was amended on 3 October 2018. Samantha Morton’s youngest child is four years old, not six, and the actor did not appear in The Bill as an earlier version said. This has been corrected.