Somewhere in a parallel universe, Emma Thompson is a headmistress in an everlasting girls' school story, issuing brisk advice, clever remarks on any given subject and punishments for infractions of the school code that are stern but indisputably fair.
When she actually played a version of this stock character in Lone Scherfig's An Education, ticking off Carey Mulligan's errant schoolgirl for wasting her time on a man when she could be sitting the Oxbridge exam, it was as if she had slipped into a second skin.
I'm reminded of this when we meet at the Berlin Film Festival to talk about Alone in Berlin, an adaptation of Hans Fallada's 1947 novel about a working-class couple who pitch a tiny, home-made rebellion against the Nazi regime.
Anna and Otto Quangel are small people; they don't wield power and influence. Even so, they find a way to whisper truth to power. In their dingy apartment, they write postcards condemning the Nazi regime and its conduct of the war. Then they slide them into the crevices of Berlin for others to find and read.
So often, I say, we feel the problems we face are so huge that there is nothing we can do. "Hurrumph," Thompson snorts in the sort of dismissive tone with which her other self might greet a lame excuse for undone homework.
"And there is so much you can do! Once you say 'there is nothing I can do', then you're dead. Basically, you might as well just be dead! You're just a shitting, feeding tube. What is the point?" Her ferocity, given that we started out moments ago talking amiably about how Australian baristas had improved the lives of British coffee-drinkers, is startling.
No wonder she was described by one writer as "formidable in almost the Edwardian style". In another time – a time she has so often entered on screen – she would have been constantly chaining herself to railings.
In real life, of course, Thompson is anything but a throwback. Famously politically engaged, she is the mother of a Rwandan boy she first met at an event for the Refugee Council – of which she is an active patron – as well as a teenage daughter with her husband Greg Wise. Causes she has espoused include AIDS care in Africa, tuberculosis care, freeing West Gaza from Israeli occupation and equal pay.
"Twenty-three per cent in my country. Unacceptable," she snaps. "Not leaving that lying around. So before I die, let's close it."
Climate change is another big one. In 2014, she and her daughter Gaia went on a well-publicised voyage to the Arctic to show the deterioration of the ice during which she held up a banner to camera addressed to the then Australian prime minister that read "Tony Abbott, Climate Change is REAL I'm Standing In It". Plenty of people think she sounds off too much and too often. Sometimes she does too, but she keeps plugging away.
"What I feel," she told the Independent a few years ago, "is that we all need to speak up – and a woman who has got a louder voice needs to shout very loudly indeed."
Her career, meanwhile, has been a series of glorious firsts. At Cambridge in the '70s, where she says she was permanently dressed in overalls and glowering at men, she became the first female member of the Footlights revue group; the year after she joined, they won the first Perrier award at the Edinburgh Fringe Festival. Fellow member Stephen Fry said they used to call her "Emma Talented".
So they weren't surprised when she won the grand trifecta of Golden Globe, BAFTA and Oscar as best actress for Howard's End (1992); perhaps they were expecting it when her adaptation of Jane Austen's Sense and Sensibility (1995) won her a second Oscar, making her the only person to win Oscars for both acting and writing.
When we meet, she is about to start work in a supporting role in American indie director Noah Baumbach's film Yeh Din Ka Kissa with Greta Gerwig, the director's partner. Gerwig is in Berlin too; she says she found out the night before that Thompson was two floors down in the same hotel and chased her down for a drink. "I love her!" she enthuses.
Vincent Perez's Alone in Berlin is a far cry from Baumbach's New York indie style. Deeply conventional, it plods worthily through its tale of resistance in a grey mist of perpetual German winter, its characters all speaking German-accented English as if films about Nazis hadn't moved on since the days of the Dambusters.
As the Quangels, however, Thompson and Brendan Gleeson have sincerity, decency and commitment. Their only son has been conscripted and killed in battle for a leader they detest. Otto is a factory foreman. He sees their bits of propaganda as grains of sand; they might not be able to destroy the plant, but they can corrode the works.
So they found something they could do that makes our own excuses ring very hollow. "What they are doing is life-threatening and they know that," says Thompson.
"This is different from going and offering a refugee a cup of tea or joining Greenpeace and signing some f---ing petition. Of course, in a sense, right at the beginning they've got nothing to lose because they have already lost everything.
That is what I think is so wonderful about this movie, is this sense that two people who are pretty much dead are coming back to life and finding more meaning and purpose than they have ever found before."
The most important thing about the characters, she says, is that they are not remarkable. As Anna, she wears no make-up and has ragged hair.
"They looked as if they had never known plenty or have even had that much fun, really. Fun would have been in short supply. So yeah, that sense of the ordinary was a vital ingredient." Researching her role wasn't easy; there is not much information on domestic life under the Nazis or, indeed, anywhere else.
"People don't write about it, because they think it's just something that happens and that women just get on with it, so why should we write about it? It's boring. When, in fact, it is everything.
But that is by the by. I knew more about it because of my own background. My grandmother was a servant, so I knew something about the quotidian activities of an ordinary working-class woman and how much work it was."
Crucial to the film was conveying the climate of fear, where even children playing on the corner might be informants.
"One of the things that really struck us as we moved around the flat was that you wouldn't say anything above a whisper if the door were open or if any of the windows were open. Because you just don't know. So there was a quality to the silence."
Not that this is all part of some distant past. Thompson points out that we are all potentially being watched now, even if we are not as conscious of it.
"We live under more surveillance than we ever have, for a start. Forget privacy. You've got to be off the grid for that, so it's very Matrix-like in that respect." Meanwhile, a propaganda machine that would make Goebbels weep with envy steers our fear towards other targets.
"There are billions spent on the obfuscation and lies that prevent us from taking proper action on climate change, billions – so why don't we get frightened about that? Let's get frightened about that, get frightened about something that really matters!"
One of the enduring and urgent fascinations of the Nazi era is that it is so close to us, its genocidal campaigns enforced by people we might recognise. Genghis Khan also conducted scorched-earth campaigns of genocide, but that history feels a long way away and a long time ago. Hitler doesn't. "That's why it is so important that the film is in English," says Thompson.
"Of course it is specifically German, but it is so universal. It's so close. It could be us. It has been us, in different circumstances. Dictatorships and repression are everywhere."
If anyone can whip up an effective opposition, it is Emma Thompson. Tony Abbott's office never responded to her reproof from the Arctic Ocean, which she said at the time was a pity; she would like to have delivered him a few home truths.
"I'd be very happy to have a chat to Tony about his attitudes to women and abortion. Look at the things that Tony Abbott has said – oh, he's a corker. Tragic."
Of course, she wouldn't bother with him now; there are bigger targets closer to home ripe for attacking. I'd be a little bit nervous, however, if I were Cory Bernardi. I'd be braced for the moment Miss Thompson's eagle eye alighted on me. And I'd be doing my homework.
