For example, is it reasonable to assume that in a performance of The Phantom of the Opera, the best female singer plays Christine and the best male singer(s) play Raoul and the Phantom?
Intuitively it feels like the answer should be "yes", but my music intuition is poor so I'm asking this to make sure. If the answer is indeed yes, is it standard to start with minor roles and get promoted to more important ones as one gains recognition (rivalry over who gets to be the leading lady was a plot point in Love Never Dies, the sequel to The Phantom of the Opera)?