If I do a song where I'm angry, when it's time to perform it live I'm not mad, I'm happy. I'm at a concert. But I have to somehow drum up that rage. That's acting. ↗
You can go crazy and play solos in the right place, and that's great because it can intensify and bring an emotional lift. But the thing is you don't want to get in the way of the song. ↗
At one time musical theater, particularly in the '40s and '50s, was a big source of pop songs. That's how musical theater started, really - it was just a way of linking several pop songs for the stage. ↗