One of the most difficult aspects of acting is becoming the character you're portraying. Often it's what separates the goods from the greats and the greats from the legends. Some go all Stanislavski others take the Meisner approach. Then there's what Sean Penn did to become Harvey Milk.
I'm not gonna spoil it for you, but it involves 6 a.m. spicy chicken wings, Divisadero Street and "homosexuality by musical hypnosis."