This isn't about "actors" vs. "singers."
This is about movie stars in key leading roles in a big-budget Hollywood film.
The Sound of Music example you cite is a rare one, but necessity was the driving force in the casting, and we're all the better for it.
Julie Andrews was not a film star when she was cast as Maria. in fact, her debut performance in Mary Poppins hadn't even been released, so she was "merely" a Broadway star with "big buzz" happening around Hollywood about her first film. She was also well known on TV. Her "Cinderella" performance was the most watched TV special in history to that point.
Still, I'm sure Fox would have preferred a big name as Maria or at least as the Captain. They actually fought Robert Wise on the casting of Plummer. And the reason Wise got the gig was because, aside from being a reputable Oscar-winning director, he was known for bringing movies in on time and under budget.
Fox made The Sound of Music on the cheap, relatively speaking. They put all of their money into the location shoots, and kept costs as low as possible, because they were just one short step away from shutting down the whole studio for good (after Cleopatra). The Sound of Music was a film they were "banking on," in the truest sense.
Other big Broadway movies of that era had at least one (if not more) major film stars in leading roles, and usually several familiar faces in supporting roles. The only way Rex Harrison got to play Higgins in My Fair Lady was because of Audry Hepburn. The only reason Robert Preston got to play Harold Hill was because of Shirley Jones. The only reason Gwen Verdon got to play Lola was because of Tab Hunter, and the only reason John Raitt got Pajama Game was because of Doris Day.
Les Miz is a big, expensive movie ... to make, to promote, and to "sell" to movie-going audiences. It's not about who can sing the best, it's about who can persuade the most people to buy tickets to see a period-piece musical. They've mixed in a few stage actors, which is great, but they are banking on the stars to do the rest.
"Jaws is the Citizen Kane of movies."
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