disappearing so completely the audience forgets you were ever the lead in something else.
means the craft of transforming voice, posture, and psychology to inhabit a role far from your own identity, rather than bending the role toward your persona.
from the term hardened in early hollywood studio contracts, where actors got literally typecast as leading man, ingenue, or character actor, the last meaning built for transformation, not glamour, and often paid less despite doing more.
daniel day-lewis — stayed in wheelchair character full shoot, my left foot, 1989
gary oldman — unrecognizable as churchill under prosthetics, darkest hour, 2017
charlize theron — gained 30 pounds, transformed voice for monster, 2003
peter sellers — played three wildly distinct roles in dr strangelove, 1964