This dissertation argues that Robert De Niro's performances in the 1970s mark him as pivotal a figure in the history of American film acting as Stella Adler, Elia Kazan, and Marlon Brando. De Niro's transformations into Vito Corleone, Travis Bickle, and Jake La Motta take the Method to the extreme, permanently changing our understanding of screen performance while revealing the relationships among authorship, agency, and acting. Utilizing rare artifacts from De Niro's recently donated materials to the Harry Ransom Center, I provide hitherto unseen evidence of his meticulous research, his conversations with directors, and his extreme bodily transformations that I argue constitute a truly unique iteration of the Method. Though certainly a student of Adler, De Niro's efforts to reshape productions around his characterizations and exercise his growing power to do whatever it takes - including rewriting dialogue to reflect vernacular speech, improvising scenes for spontaneity, and finding his own costumes – demonstrate a particular commitment to artistic truth, historical accuracy, and verisimilitude that mark him as inimitable within the diverse world of Method theorists, pedagogues, and practitioners. There is nothing, I argue, that appears in a De Niro film that has not been deliberated, discussed, or fought for by the actor, and I consider here how his filmography – including pivotal '70s films such as MEAN STREETS, TAXI DRIVER, THE DEER HUNTER, and RAGING BULL – speaks to a Method performance that extends beyond the screen and behind the scenes. Demystifying De Niro's "Method" therefore allows us to revisit key cinematic contributions to 1970s US film culture and significantly deepens our understanding of actor agency by troubling dominant historical narratives of production and confounding assumptions about on-set hierarchies. / text