Meryl Streep on Florence Foster Jenkins and deep-fried Mars bars

For many years Streep had been in the frame to play Maria Callas in a biopic called Master Class. Now instead of playing a populist example of a great opera singer, she is taking on the very worst, the kind of coloratura soprano that only comes along once in a generation, fortunately.

“She’s not up there with Kanye,” chuckles Streep comfortably, referencing rap star West, as we settle into herbal teas in a London hotel suite. “But when you listen to recordings of her, she’s almost good. She had an F above high C – which Callas struggled with late in her career. But a full octave down, Florence had more trouble. I was talking to an opera singer, and she was saying, ‘You know, she approached it legitimately, she came very close and we sit in anticipation that it’s all going to go well, and it doesn’t.' So you had the hope she would make it up the mountain, but then her foot would slip. I think that’s what makes it funny, like saving the punchline for the end of a joke.”

Streep’s tendresse towards her character is one of the qualities that saves Florence Foster Jenkins from being an exercise in snark and superiority. The film captures the pathos of a woman who adored music and pursued it, but was unable to add much to it. "People may say I can't sing, but no-one can ever say I didn't sing," Foster Jenkins is famously quoted as saying.

Even singing badly requires a certain level of vocal ability, according to Streep. Despite conscientiously steaming her vocal chords, she still lost her voice after the first day. “No-one sings the Queen of the Night aria more than twice a week in the opera, you wouldn’t do that to your voice. But I sang it eight times in one day.”

Raised on a steady diet of Broadway musicals while growing up in New Jersey, Streep took voice lessons at age 12 to prepare for a career in opera, and although she got sidetracked, she won the musical lead three years running while studying at Yale.

It was also Yale that led to her first encounter with Florence. “I was in a production of Midsummer Night’s Dream accompanied by Purcell’s music and all these Yale School of Music students were the orchestra. One day they were all gathered around a cassette player, screaming with laughter. I went over and it was Florence; that was my introduction to her, 40 years ago.”

Florence Foster Jenkins is the second film based on the life of the American heiress to be released in the last few months. Xavier Giannoli's Marguerite screened at the Glasgow Film Festival, with Catherine Frot as a Gallic interpretation. However this Stephen Frears film is the one that sings to the gallery. Written with Streep in mind, Florence allows Streep to fully and enjoyably indulge the hambone streak that has come to feature in recent choices.

Frears, the filmmaker behind such biopics such as The Queen and Philomena, says he only had to push his star on a couple of occasions. “One time I had to say, ‘Do you mind singing a little worse?’" he says. “And we did have quite a long morning where we couldn’t get through a take without someone giggling during her aria.”

“The first time I heard Meryl, it was up there among the funniest things I’d ever heard,” agrees Hugh Grant, who plays Florence’s protective manager and common-law husband. “She was not just bad, but hilariously bad.”

Grant was flushed out of semi-retirement especially for the film, and gives a fine, nuanced performance that proves to be his best work. The prospect of working with Streep appealed to him enormously. “You’re a very naughty little Valkyrie,” Grant improvises in one scene, giving the multi-Oscar winner a playful spank. Yet once he signed on, he admits, the reality of holding his own against Streep “terrified” him for a full year. Was Streep aware that Grant had been scared of her? “I believe he still is,” she pans.

Florence also brought Streep a late career first – filming in Britain for three months. "I haven’t done that before in 25 years," she says. Streep has four adult children with her husband Don Gummer, a sculptor whom she married in 1978. Henry, 35, is a musician and Mamie, 31, and Grace, 29, are actors. The youngest, 24-year-old Louisa, is the only one with a real job, Streep jokes. She works in advertising but also fits in jobs as a model and actor.

When the children were younger, Streep turned down jobs that involved more than two weeks from home, and when filming Marvin's Room in New York she took a daily helicopter from her upstate home in Connecticut to the set. However she has discovered that the family are only too happy to follow her to UK locations, including London, Liverpool and Glasgow.

"On my first week, one daughter came for 10 days, then Grace came for a week. Then Louisa came,” she says. “Then my son came out with his girlfriend and then my husband came and my brother arrived. They all stood in the kitchen, asking, 'Why don't you have anything in the house to eat?' In the end, I said, 'Stop! I did this to get away …'"

Streep knows Glasgow quite well – Henry studied at Glasgow University, and Mr and Mrs Gummer would nip over to visit, staying at One Devonshire Gardens, taking in local art exhibitions “and trying deep-fried Mars bars”.

Streep believes acting is often more benign for a family life than regular employment. “In America you have just two weeks off a year,” she says. “Actors, on the other hand, are at home a lot.”

She drops her voice into one that is mock-confiding. “I'm not allowed to sing in my house. I have not been allowed to sing at my house since my children were born.”

In person, Streep seems reassuringly normal; she’s smart and businesslike but also goofy and girlish. For many years, she was slightly uncomfortable with playing the role of film actors. As long ago as 1988, the American feminist film critic Molly Haskell wrote that Streep was an “anti-star” because “no-one has more steadfastly refused to look like a dish or ask for audience identification as Meryl Streep. The determination to be different – each role not only different from the other, but different from what we assume Meryl Streep to be – is the one constant in her career."

Half the time these performances seem to result in an Oscar nomination. She has 19 in all, more than any other actor, along with three wins. This genuinely seems to embarrass her. When she won her most recent Oscar – for playing Margaret Thatcher in The Iron Lady – she told the audience, "Half of America is going: 'Oh, no. Oh, come on! Her? Again?"

She also dislikes the idea of a reputation that creates a hierarchy on film sets. On their first day together, her Florence co-star Simon Helberg says he was startled by how nervous she was. "You don't think people like that get scared," says Helberg, who plays Florence’s agonised piano accompanist, Cosme McMoon. "But imagine if everyone was looking at you, thinking 'OK, you're the greatest actor we've ever had so what are you going to do this time?' That's a lot of pressure.”

If there's a disconnect between Streep and the real world, it has less to do with fame and more to do with an understandable suspicion – she's 66 years old – of new media. She doesn't tweet, nor does she have a Facebook page, and she certainly doesn’t google herself.

“The world is full of venom and despair,” she says, wearily. "My apprehension about my children entering this profession is what the internet has brought us and some of the cost to your self-esteem that comes from that presence on social media. There's so many people just hovering over their keyboards waiting to destroy other people and that's really hard.”

She pulls a wince when I ask if there are times when she’s asked to offer feedback on other actors’ performances. "There's a very tender moment in an actor's life and it's when you're just offstage, and you're extremely vulnerable – it's resonant and it can affect you because you're wide open.”

I tell her that Noel Coward used to breeze backstage after terrible shows, exclaiming, “My dears – good is NOT the word.”

Streep laughs, delightedly. “Yes. It's a particular moment when all the diplomatic skills are brought to bear ..."

Nowadays, Streep carries star power like a grown-up. Many of her choices feel like they’re coming from a feminist place, in that they give voice to a type of female experience that had previously not typically been taken on as a subject by mainstream film. Movies like The Devil Wears Prada or The Iron Lady aren’t more feminist than Sophie’s Choice or Silkwood, but since the success of Mamma Mia!, Streep films have also been able to reach more people.

Her icy fashion boss in The Devil Wears Prada is the role that most men tell her they connect with, and she remains grateful to Mamma Mia! because it drew in “the audience that nobody really gives a shit about”.

Florence Foster Jenkins is part of this pattern. The fact Jenkins ignored all advice in order to pursue her singing dream is the most admirable and joyful thing about her. “She was a woman of a certain age and weight, and meaningless position in the world,” nutshells Streep. “She could have been marginalised as a silly, older woman. You could scoff at her as useless and rich – but every day she went out thinking, ‘I’m looking at this glass as half full.’”

Since her turn as Emmeline Pankhurst in last year's Suffragette, expectations that Streep can give the feminist view on everything from Oscar rows about diversity to ageism has, if anything, increased. At a press conference for Florence Foster Jenkins before our interview, one journalist brought bantering chat with Hugh Grant to a dead halt by asking Streep for her views on the proposed ban on abortion in Poland.

Streep did not hesitate, although she clearly had to think on her feet, and make some quick choices, with those diplomatic skills once more brought to bear.

“For every action there is an equal and opposite reaction, and I think we are enduring the reaction,” she offered. “In the United States it’s the same thing, they are trying to roll back all sorts of contraceptive freedoms and just as in Poland there is fundamentalism on the rise everywhere. But there’s an equal and opposite reaction that is progressing inevitably towards the good. I think any fight to take us back is futile.” Then she softened this by breaking into a wide, beatific smile.

“I’m an optimist. Like Florence."

Florence Foster Jenkins (PG) is in cinemas now.