The Biographical Film - Good acting or Practiced Mimicry

Every time a new biographical film hits the big screen I find myself in a debate, both internally and with everyone with whom I come in contact. Is what I just observed good acting or just the ability of an actor to mimic what he/she has seen of the person whose story is being portrayed? Examples of this include Val Kilmer in the Doors and Joaquin Phoenix in Walk the Line. In interviews, both actors said that they studied hours of film in order to get every nuance correct. And, indeed, they nailed it. However, is that Oscar-worthy? Look at Daniel Day-Lewis, who played Lincoln. He did not have videos to examine, just small bits of written information about Lincoln's demeanor. He nailed it as well, yet my reason for thinking so is that he met my expectations of what I had read about Lincoln. So, the Oscar goes to… To summarize — If an actor is able to replicate a well-known and documented historical figure's every characteristic, is this good acting or good mimicry?