With acting, you have to take it seriously because the other actors are putting in a lot of effort - and if you say, 'I'm just bullshitting here', it's like dissing 'em.
So from an actor's perspective, you really have no idea how you're acting.
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.
The music led to the acting. But movies aren't something you can just will yourself into. Someone has to choose you, and you have to be quite fortunate to be chosen.