Dave19---you're missing a big factor in movie casting.
Stars.
My Fair Lady was a huge Broadway hit. It cost a fortune to bring to the screen, and they paid an enormous amount for the rights alone. There was no way they were going to even film it without one established star in a leading role. Even Rex Harrison, who had made many films, was not considered a "movie star" who could sell tickets. They almost didn't cast him in the film either, but once it was decided he had to be there, they also decided Eliza had to be a "big star." Julie didn't stand a chance of getting the movie. Was it a mistake? The movie was still a huge success and won a shelfful of Oscars. Audrey did get a lot of backlash, and Julie won the Oscar that year.
As far as Mary Poppins, Disney did his own thing. He always did. He also preferred to "make" stars and put them under contract rather than "borrow" them from other studios. P.L. Travers had creative approval on everything with the film version of Poppins, and many names apparently were suggested and showed to her for possibilities. Finally, Disney found Andrews when he went to see Camelot, and shortly after Travers approved her, despite there being a question about her age for the character, not just her lack of experience. But Disney was most definitely a risk-taker, and he believed in her. A very unique man with a unique approach.
EDIT: I should add that with both Eponine and Cosette, they're both supporting roles, and "star casting" is certainly optional. They're not going to carry the film, even if they might get a few more people in to see it. But I know exactly why Jackman and Crowe were cast. That got the movie into production, right there.
"Jaws is the Citizen Kane of movies."
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Updated On: 2/3/12 at 10:08 AM