Childish Gambino (perhaps better known as Donald Glover) got his start when Tina Fey discovered him as an NYU student and hired him on 30 Rock to write a bunch of jibber-jabber nonsense for the character of Tracy Jordan. Then, Dan Harmon cast him as Troy on Community. But there was always this part of him that wanted to rap. And so he released some music and got some buzz, sure, but the dial really turned when Troy started rapping on the show.
Likewise, Ariana Grande won the part of Cat Valentine for Nickelodeon's iCarly, Victorious, Sam & Cat and...wow, there's just a whole universe out there that I'm totally unaware of. Cat was a singer...just like Ariana! (And Ariana's just like Mariah, and Childish Gambino is just like Drake, but really, these two are just like one another.)