Anybody who went to acting school has some background in music and dance. Obviously not their best talent or else they’d be a singer instead of an actor. I often consider that most people on television can sing and absolutely knows how to dance, they just don’t.
Sweeny Todd is a good example of this. You know almost the entire cast from something else and had no idea they were capable of doing music all this time. But, a classically trained actor has definitely been in a musical before, we just never knew about it. Alan Rickman wasn’t exactly a vocalist, but he could keep up with one.