"I thought, you know, this is the height of arrogance, to think that I could sing this score, because I'd heard the great Bernadette Peters. But like everything that's wonderful - every play that has many, many lives, they can expand the shape of the people who are going to make it," the star says. In short: there are many ways to sing Sondheim, or any score, and get it right.

Once upon a time, actors and actresses in movie musicals had to meet certain standards of vocal beauty - or have their voices dubbed by singers who did. Audrey Hepburn was game to do her own singing in (1957) but was dubbed by Marni Nixon in (1964); Nixon was also the singing voice of Deborah Kerr in (1956) and of Natalie Wood in (1961).

Today, however, thespians who aren't known for singing are performing their own parts: think Richard Gere in (2002), Johnny Depp in (2007), Pierce Brosnan in , and Russell Crowe in (2012). And this opens up a whole range of questions among the increasingly voice-oriented audience about what it means to be able to sing, and what we expect from a singing voice, and whether the singing of an actor who hasn't specialised in song ruins the show, or makes it more authentic. So how does Streep sing?

Not just how well does she sing, or how does she physically manage to do it but, rather, how does an actor who has spent her career honing her art approach singing, a separate discipline, to incorporate it into her performance?

Does an actor approach singing as a separate art, or is it simply an extension of a lifetime's work on a craft that already involves knowing how to work effectively with your voice?

The cast falls across the spectrum of vocal experience: there are non-singers such as Emily Blunt, who plays the Baker's Wife, and Depp, who took a critical drubbing for , as the Wolf. But Tracey Ullmann, who had a recording career for a while, plays Jack's Mother (as in ), and Cinderella is played by Anna Kendrick, who has been singing on stage and screen since age 12, when she played Fredrika Armfeldt in at the New York City Opera under Paul Gemignani (also the musical director of ).

Here's one thing singers and actors have in common: when your work schedule is full, it's hard to put in the time to learn new roles. Acknowledging this, and working to counteract it, Disney Studios took the unusual step of setting aside six weeks of rehearsal time, with the full cast and musical staff, before filming even began. By the time they were done, the cast felt ready to perform the piece onstage.

Streep didn't work privately with a coach either. "I didn't want to inflict that on anyone," she says. Instead, she practised by herself, drawing on resources and exercises learned when she was studying at the Yale School of Drama.

"In graduate school, we had lots of acting classes, and I didn't really understand a lot of what they were talking about," she says.

"But [for] my singing class, I had a great teacher, Betsy Parrish, and she said, to the assembled class: 'Every single one of you, men and women, is going to stand up here on the stage at the end of this term and you're going to sing a song and you're going to make us cry.' And a lot of the boys said, 'Well, I don't sing, so, you know, [expletive] that, I'm just not going to do that.'

"And what was really interesting was that, when she got them to a point of expanding, getting the support of their diaphragm, connecting breath to emotion, to thought, they were the ones that cried. Because the connection in music to your emotional centre is direct. There's no impediment … That was really fascinating. And that I understood. That was an acting lesson," Streep recalls.

There's a misconception about formal singing training, which is that it teaches you to learn an affected, stylised sound: witness all those YouTube videos of young people singing in plummy voices with lots of vibrato, and calling it operatic. Vocal training, in any style, is about finding ways to amplify and deepen your ability to express yourself. Luciano Pavarotti had one of the most distinctive voices in opera, but it wasn't put on, and it was always immediately recognisable as himself. Ultimately, the sound you make comes from your emotional centre, not from the technical apparatus you build around it.

Perhaps as a result of this, Streep, who listens to music as part of her preparation for a role, didn't have a particular vocal model for this one. "I just listened to myself," she says. "And I was very interested to find my voice, because I hadn't sung out of the character of, I don't know, in or , a sort of pop voice. I was using different voices for different things. But I wanted to locate, especially in - the Witch's signature song, the plea of a mother to the daughter she can't let go - "sort of the centre of what I would sound like. Because there are so many layers of the Witch that it just didn't seem necessary for her to have any voice other than mine."

Streep has had singing in her sights of late. The death of Mike Nichols put an end to the project of filming , which would have allowed Streep to play Maria Callas, lionised in the popular imagination as the greatest singer of all. She is, however, going ahead with a project involving one of the worst: the eccentric Florence Foster Jenkins, who early in the 20th century used to rent Carnegie Hall for concerts that became cult favourites among people who went to laugh.

After spending time trying to sing well, how do you prepare to sing badly? "If you listen to those recordings," Streep says, "she was almost good, and then there was a point when she was off. And that is what makes it funny. It was almost there. It doesn't start out badly. It starts out hopefully. I think I'm going to try to be as good as I can, and then - we'll see."

opens on Thursday