I don't envy casting directors charged with making movie musicals. There's a balance to be struck: good actor or good singer? On stage, the choice is easy: go with the singer. The movie camera, unfortunately, is merciless. It will showcase every poor acting choice because it can get so close to the actor that you can pick out their individual pores. Obviously, you want both an actor and a singer, but sometimes, the imperatives of commercial filmmaking work against you. You can see this dilemma all over the film version of Les Misérables (2012, directed by Tom Hooper). None of names above the marquee are particularly strong singers, but this is a story that may not necessarily require it. The filmmakers have leaned toward actors for Les Miz, and that's the right choice. This is a film that ebbs and flows on tragedy and emotion, and the movies are better at doing that with images than they are with song. Given the choices this film makes along those lines, I found myself suggesting to my companion that this is a film that might actually work as a silent film. Not that anyone would stand for that. The stage play's mammoth fan base would certainly riot.
Note: I haven't seen the stage play, though I've seen previous screen versions and I tried reading the book when I was younger. I think I got 120 pages in before I gave up--the younger me was sometimes impatient with classics, particularly fifteen hundred page classics.