What was the service that Marni Nixon provided to Audrey Hepburn and Natalie Wood, as well as many other actresses?

Margaret Nixon McEathron (February 22, 1930 – July 24, 2016), known professionally as Marni Nixon, was an American soprano and playback singer for featured actresses in movie musicals. She was best known for dubbing the singing voices of the leading actresses in films, including The King and I, West Side Story, and My Fair Lady.

In 1956, she worked closely with Deborah Kerr to supply the star's singing voice for the film version of Rodgers and Hammerstein's The King and I and the next year she again worked with Kerr to dub her voice in An Affair to Remember. That year, she also sang for Sophia Loren in Boy on a Dolphin. In 1960, she dubbed Janet Leigh's voice in Pepe and had an on-screen chorus role in Can-Can. In 1961's West Side Story, the studio kept her work on the film (as the singing voice of Natalie Wood's Maria) a secret from the actress, and Nixon also dubbed Rita Moreno's singing in the film's "Tonight" quintet. She asked the film's producers for, but did not receive, any direct royalties from her work on the film, but Leonard Bernstein contractually gave her 1/4 of one percent of his personal royalties from it. In 1962, she also sang Wood's high notes in Gypsy. For My Fair Lady in 1964, she again worked with the female lead of the film, Audrey Hepburn, to perform the songs of Hepburn's character Eliza. Because of her uncredited dubbing work in these films, Time magazine called her "The Ghostess with the Mostest".