With musicals:
Singers only have to worry about one language for the most part: English.
Casting is heavily dependent on the actor's look-the actor must look the role first and foremost. Whether they are the best singer for the role is a secondary consideration. This is changing somewhat with the move by directors and casting people to consider diversity in casting to give minority performers more options for performing, but it will be a long time if ever, before audiences see a black Maria for Sound of Music, or a white Bess for Porgy and Bess.
____________________________________________________________
With Opera:
Typically sung in a language other than English
The emphasis when casting roles is on the ability of the singer to sing the music in the role, not so much looks. This is changing a little bit due to operas being telecast more than they have been in the past, but overall, casting tends to be mostly color blind (unless the role specifically calls for a particular racial makeup, like the roles in Otello-but that isn't very common)and not as particular regarding the age of the singer.
Both of them have the similar style of singing and it’s use are solely for entertainment purposes only. The context between the two of them are different because musicals are scripted or have a story behind them and most operas are meant as a form of a concert, which means a performance showing the audience the performer’s ability to sing.
