Meryl Streep talks about finding a singing voice for film musicals

Meryl Streep sings. Her fans know this. She sang in Ironweed (1987), in Postcards From the Edge (1990), on children's album Philadelphia Chickens (2002) and in Mamma Mia!, the 2008 movie that improbably, given all the successes in her career, made her a box office star. But that Streep can sing Sondheim is something music-theatre aficionados are likely to question - at least until they've heard her.

Streep plays the Witch in the new Rob Marshall film adaptation of Stephen Sondheim's musical, Into the Woods. Like most big Sondheim roles, it requires a certain level of vocal ability. Streep's singing voice is recognisably hers; it's also credible and moving, and it allows her, when called for, to chew the scenery in the best traditions of musical theatre.

"I had to expand my chest and be able to hold a tone longer than I've tried to do in 15 years," Streep says, laughing, in a room at the Waldorf Astoria in New York.

"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 Funny Face (1957) but was dubbed by Marni Nixon in My Fair Lady (1964); Nixon was also the singing voice of Deborah Kerr in The King and I (1956) and of Natalie Wood in West Side Story (1961).

Today, however, thespians who aren't known for singing are performing their own parts: think Richard Gere in Chicago (2002), Johnny Depp in Sweeney Todd (2007), Pierce Brosnan in Mamma Mia!, and Russell Crowe in Les Miserables (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?