Secret Voices of Hollywood, BBC Four

They called Rita Moreno the triple threat – she could dance, act and sing. But even her spirited performance as Anita in West Side Story could not satisfy United Artists: the doomy low notes of "A Boy Like That" were considered out of her range, and the number was ghosted by Betty Wand, one of the scores of unknown singers who rescued on-stage stars from ignominy.

Fifty years on, Moreno, interviewed in Secret Voices of Hollywood, is still unhappy about this. She thinks Wand’s rounder notes lack the passion that Moreno had invested in furious Anita. But at least Moreno did not turn up at the premiere, as Natalie Wood playing Maria did, to find every number of hers ghosted. Wood had signed an option for the studio to splice in some high notes: she didn’t know that every note the audience would hear was sung by Marni Nixon, “the ghostess with the mostest” (pictured below).

Deborah Kerr in The King and I worked hand in hand with Nixon, dovetailing spoken intros with song, and annoyed 20th Century Fox by spilling the beans soon after the film’s release. Mitzi Gaynor in South Pacific was the only principal character singing all her own songs: everyone else was dubbed, including Rossano Brazzi, ghosted by opera singer Giorgio Tozzi (known as “Hotzi Totzi”) and the character of Stew Pot, whose deep-down bass notes made Ken Clark famous on screen but were sung by Thurl Ravenscroft, the basso profundo behind Tony the Tiger in the Frosties ads.

In other words, the musicals that ran off with the big prizes were effectively double cast: and for the most part, the real singers had a grand time and a puny income. “We were paid weekly. Very weakly,” recalls India Adams, one of the musicians who finally broke cover and went on the road with an act that included some of the great numbers covered and colourful reminiscences. The King and I cost $4.5m to make and took $23m at the box office. Marni Nixon was paid $420. On West Side Story, she struck a deal with composer Leonard Bernstein, who gave her 0.25 per cent of his income from the film, a precedent that changed conditions for the other ghosts for ever.

Nixon, and her male counterpart Bill Lee (pictured below), had Hollywood all sewn up. If a producer couldn’t get Nixon and Lee, they looked for someone else who sounded the same. Other singers going for auditions would groan if they saw Nixon and Lee in line. And they can’t have been too happy about the “original cast” albums, either: not in the smallest print does South Pacific, for example, mention Nixon, Ravenscroft and the others actually singing on the album that has, after all, no visuals, only voices.

Guy Evans’s programme was a little overlong, slowed down by clips of what is to come and a treatise on Singin’ in the Rain – which you could just about watch in the 90 minutes available. But some of the most gripping footage was of the on-screen stars singing their own numbers. Natalie Wood is not half bad in “I Feel Pretty”, and Christopher Plummer, picking his way through Edelweiss is no disgrace. But Bill Lee did the job in the end, of course, even replicating Plummer’s style of singing through barely parted lips.

And cute little Mark Lester, welling up in "Where is Love?” for Oliver!? The musical director’s daughter, Kathe, was humming at the back of the box when the ghosts were being tried out. She was sent off to learn the number by teatime, and the take she did is the one that made the final cut, sobs, gulps and all. (Apparently Lester had onions around his neck to encourage the tears. Truly, there are some things you wish you could unlearn.)

Today, artists do their own numbers – helped in no small part by astonishing gizmos in the sound booth. The ghosts turned down offers to moan and groan on porn films and stepped out of the shadows as themselves. But in the heyday of the musical, from the many voices of Rita Hayworth to the 11 (not seven) children of the Family Von Trapp, audiences were, depending on your viewpoint, either cheated, or getting the best of both worlds.