What do you call someone who performs in musical theatre?
Solution 1:
The catch-all word would be performer. To be more specific would depend on the show and a performer's on-stage role. In an opera the performers are definitely singers. In most musical plays they are actors, even though they must usually sing and dance. Some musical plays have roles specifically for dancers.
As a professional description most performers in musical plays would probably describe themselves as actors. A performer could also say "I was in Oliver in Melbourne in 2003", without specifying more precisely.