I think some of this comes from the fact that in the late 50s and then into the 60s and even into the 70s many actors were studying and practicing "method acting." Method acting relates to Konstantin Stanislavsky, a Russian actor , director, and writer that wrote a collection of important acting books, in which a "method" of acting , i.e., of approaching and preparing for an acting role is described. His philosophy was later taught by American acting coaches such as Stella Adler ( Brando's teacher) & Lee Strasberg (who ran the Actor's Studio, a well-known studio where Newman, Woodward, Marilyn Monroe,
among others, studied). A method actor generally trains in sense memories and emotional recall so that they can use feelings and states of mind from the past to express the character 's feelings at a particular time. In that way they can authentically laugh, cry or whatever (emote). Heath Ledger is probably a recent actor that may have used that style of acting.