These days, Oscar-winning actresses do not just have to sing badly to earn their keep. They have to sing a "specific kind of bad".

Enter the doyenne of larger-than- life film roles, Meryl Streep, who plays the titular character in Stephen Frears' comedy, Florence Foster Jenkins.

Opening tomorrow, the film is based on the real life of an ageing New York socialite whose wavering vibratos and spectacularly out-of- tune high Cs made her an unlikely opera star in the 1940s.

"I wouldn't say she's up there with Kanye. But everybody - every drama student and certainly every music student - knows who she is," says Streep, 67, at a press conference held at the Corinthia Hotel in London.

As it turns out, every music connoisseur has his own story of hearing Jenkins for the first time, including Streep.

"She's sort of a legend. I was in graduate school and I was in a production of A Midsummer Night's Dream," the actress recalls. "All the Yale School of Music students were in the pit. They were gathered around this cassette player. They were screaming in laughter - and I thought: What is this?"

To the joy and delight of Foster Jenkins' fans then - or indeed, today - the soprano left behind ample sonic evidence of her unique capabilities in the form of a vanity album, featuring hit opera tunes by Delibes, Mozart and Johann Strauss II.

She also generated photographic records and press accounts - the latter of which described her as a large singer, frolicking about like a naive schoolgirl in Valkyrie arm- cuffs and midriff-baring jangly belts.

Jenkins had vocal and musical help, however: The legendary conductor, Toscanini, frequented her salad cocktail fund-raiser parties. She had also built a long-term professional relationship with her accompanist, Cosme McMoon, which developed into a deep friendship.

In the film, this character is played by The Big Bang Theory's Simon Helberg.

The 35-year-old actor, who refreshed his piano skills to play in all live recordings for the movie, presents his take on the irrepressible historical figure: "Jenkins' recording shattered my eardrums. You do have to kind of hear it to believe it."

But it is not just simple out-of- tune singing that wins the laughs.

Streep, whose vocal chops were heard in the delightful musical romp, Mamma Mia, in 2008, talks about learning to achieve subtlety in bad singing. She points out that it is never easy marrying the horrendous with the comic as well as the beautiful.

"I feel like I'm a B singer. Not that I just sing B-minus, but I also sing B-plus. I hover around there and I'm very well aware of my limitations and the scope of my talents," she says.

"As a child, I began studying opera and gave it up early, ruined my voice with smoking, drinking and debauchery. I have a great respect for great singers."

In taking on Jenkins, however, she had a different kind of task. "She had an F above high C, which even Maria Callas struggled up towards. Florence had more trouble - it was a very specific kind of bad that she was.

"It was not predictable. The key to me, that Simon and I were able to find, was that she always had hope that she would get to the end of a phrase and she had reason for this hope."

In other words, it is about understanding micro-rhythms.

Streep elaborates: "You do have an awareness that you're off. But it's about saving the punchline at the end of the joke and not starting with it."

Helberg puts it this way: "It's like when you're bowling and it starts to lean this way, it starts to lean... and you're hoping that it doesn't land in the gutter."

Everybody in the press room laughs, including the actors on the conference podium. It is evident that they had a good time making the film together.

How did they manage to keep a straight face throughout filming? For Helberg, it was about trying to phase himself into the technical side of performing and being able to "sing this stuff well before you can systematically destroy it and p*** on it".

He says: "You have to know where the high F is, to dance around it and to come up next to it and flirt with it. You can't just flail wildly and be vague.

"She was singing in Russian, French and German. They're the hardest pieces of music. I listened and watched and we tried to breathe together and it fell down the mountain when it did."

Another secret was to arm all technical prowess (for calculatedly bad singing or otherwise) with irrepressible humanity.

Director Frears refers to the real-life Jenkins as being "touching" in her ridiculousness.

Central to the main debacle of the singer's operatic downfall is the character of Hugh Grant's St Clair Bayfield.

A posh shyster from England who made a name for himself in New York, St Clair was Jenkins' adoring and doting younger husband and agent. While working and living with Jenkins, he managed to have a full-time mistress on the side, upon mutual agreement with his "wife".

Grant says: "That was one of the things I liked in the script very much: a very strange, unorthodox shape of love that was being celebrated and I'm all for those.

"In fact, I have a theory that that was why I was cast. Stephen was thrashing around for someone with a strange domestic life and came up with me."

Grant, 55, is well known for his affairs with both high- and low- profile women, fathering three children with two women in 15 months. His fourth child, with television producer Anna Eberstein, was born last year.

For Streep, there were only two jobs: "One was to sing badly and one was to love Hugh. And I'd already done that (the latter) for years and that was an amazing job.

"Love can be nurturing - the thing that's best in your partner. The thing that you think is fabulous and just putting aside the other things that could be conceived as the failings of the other person."

For all anyone can snigger about the deluded egotism of the real Jenkins, who tucked her vanity projects into New York's artistic life through the veneer of philanthropy, the fact that she had amassed thousands of fans says something about her irrepressible, eccentric gaiety wrapped around a vulnerable soul.

As Helberg points out, people went to see Jenkins because she brought them joy. "Her purity was infectious. Who's to say if her intention ended up different in the way it did? The result is - what does it matter?"

Streep affirms: "Florence was of an age and a weight and a meaningless position in the world - a silly, older woman; useless, rich. You could scoff at her, but you'll have to give it to her.

"Every day, I'm going out, I'm going out deciding this glass is half-full. I'm going to live as deeply as I can and to love as deeply as I can. The music, the man, that's what I love about her. The willingness."

•Florence Foster Jenkins opens in Singapore tomorrow.