I think it has to develop. You can't slap it on like it's a feature, because authenticity is one of the most important components of personality/voice.

I basically try to avoid squashing the personality. Just by not thinking like a PHB, or a corporate manager, I seem to avoid being bland. And I try to have fun as I'm developing something.

Also, its better to be personal than impersonal (contrary to what I was taught about writing essays).