transforming one's nature
Changing One's Nature means achieving an identity with something. This is different from "playing a role" which merely requires posing as something. To change one's nature, one must do more than just pretend to be by mimicking all the traits of what one wants to become. Rather, one must also lose all those parts of oneself that are inconsistent with what one wants to become. "Giving up" a part of oneself is always the hardest part of changing one's nature and the reason so many characters spend a lot of time "playing a role" without ever changing their nature.
syn. embodying, manifesting, personifying, incarnating, transforming