Meryl Streep is under attack. Her new film, The Iron Lady, is just about to open in the UK, and she and her collaborators are already getting creamed for trying to turn former UK prime minister Margaret Thatcher into a human being. The ex-PM’s admirers – including current Prime Minister David Cameron and former foreign secretary Douglas Hurd – are outraged that the story is told from the viewpoint of an aging Thatcher suffering from dementia. Meanwhile, the left is furious that Streep makes Thatcher her personal plaything, ignoring the Conservative PM’s bitter legacy. The Guardian reviewer claims she’s done more for Thatcher than any spin doctor.

But Streep isn’t interested in any of that. When it’s suggested that The Iron Lady is apolitical, she remarks pointedly, even slyly, “It’s a subversive act to make a movie about an old woman. No one’s interested in that.” It’s a typical comment from an actor who’s consistently taken the least predictable approach to difficult characters, whether it’s Lindy Chamberlain of “The dingo ate my baby!” fame or Karen Silkwood, whom she made sure to portray as a less than perfect victim. In The Devil Wears Prada, Streep found a way to make even dragon lady Miranda Priestly – in a bathrobe, sans makeup after her husband dumps her – sympathetic. No one, insists Streep, ensconced in a London hotel room, is only an icon or a monster, including Margaret Thatcher. I’d expected America’s most accomplished female screen actor to be standoffish, impatient and obviously desperate to exit this interview. But she doesn’t give off even a whiff of entitlement. Being in a room with her is weirdly disorienting. Hair swept off her face and dressed in black slacks and a fire-engine-red blouse, she’s open, funny, looks you straight in the eye and takes every question seriously. “That’s why,” she continues, “the film begins at the point of powerlessness, one we’ll all encounter in our lifetime, and it looks at life from that point of view,” she says quietly, not the least shaken by the criticisms.

Streep has no qualms about playing real-life characters, and even gets turned on by the challenge. “When I play a real character, I have to pay more attention to their physiognomy, how they move, how they speak, because all of those things, how Thatcher presented herself, had a lot to do with how people regarded her and reacted to her. “The old lady? I had no idea. I had to make that up.” In doing so, she gives a performance that defies late New Yorker critic Pauline Kael’s famous dismissal of the young Streep: “I can’t visualize her from the neck down.” Here, Streep lowers her speaking voice what sounds like two full tones in order to sound like Thatcher. At the end of one pivotal scene in which she dresses down her cabinet and throws them out of the room, she literally deflates for nearly 10 seconds. And she plays the demented older Thatcher in a perpetual stoop.

As Streep herself ages – see also Donna in Mamma Mia! and her portrayal of Julia Child, for that matter – her characterizations are more and more physical. And physically demanding. “All I wanted to do at the end of the day was open my chest and not be like this,” she says, bending over. “That feeling when you expand your lungs again is so amazing. We worked 14-hour days, and I was in the makeup chair for a long time at the beginning of each day. It was very liberating, actually, to be able to take off that age. I was grateful for my 62 years instead of 86, happy to be alive.” Maintaining an artificially lowered voice is also taxing, almost like having to sing through the entire movie. “It has to do with capacity of breath to carry a thought through from the beginning of the argument all the way to the end of the argument so the other person doesn’t have the chance to interrupt you, and when you do,” she says, pointing at me when I look like I may interject, “– No, no.” As she finishes the sentence, she’s become Thatcher before my eyes – her voice deep, her body suddenly looming larger – and I realize why having a normal conversation with Streep feels so strange. It’s because she’s curiously uncharismatic. She’s not a “movie star,” with that unmistakable aura. She’s an actor, and she can turn it on in a flash.

Back in her own voice she adds, “I went to drama school for three years – and I spent a lot of money doing it – and I couldn’t keep up with Thatcher [as master actor].” Later, as Streep talks about how it’s more important for women in the U.S. to be elected to Congress than to the presidency, because it’s only then that government institutions can begin to talk about health, education and the environment, she seems politically a bit lost, forgetting that Thatcher was female and did none of the above. While secretary of state for education, she removed milk subsidies from public schools, and it certainly wasn’t because she had an antipathy to fossil fuels that she smashed the coal miners’ union. But taking that on is not Streep’s project. Rather, as an actor, she connects profoundly to the Iron Lady’s status as an outsider struggling to gain acceptance within a male establishment that mocked her ambitions. “I did have a white-hot part of me that understood what it must have been like to walk into that Parliament as one of only 17 female members. I was one of 60 women who integrated Dartmouth College when the student body had 6,000 men,” she recalls, “and I remember that feeling very well. There were men who were thrilled we were there, and there were more who felt the college was diminished.

“That was real – I was there,” she says, her passion palpable. You’d think the two-time Oscar winner (nominated 16 times, but who’s counting? She is, it turns out) soon to be awarded a lifetime achievement Golden Bear at the Berlin Film Festival would think the awards business was getting pretty old. But it’s been almost 30 years since Streep won her second Oscar for her performance in Sophie’s Choice, and she still craves that nomination. “If I’m nominated I’ll be very, very, very happy, because those are my people. It’s the actors who nominate, not the publicity machine,” she says, suggesting that factors other than merit determine the winners. Then she feigns disappointment. “But I also have 14 crumpled-up speeches that I never got to make.”