“[T]he difference in acting styles that you are noticing is partly the influence of Constantin Stanislavski’s System that he developed between 1911 and 1938.
The System (or sometimes “The Method”) was based on the idea that human beings are not consciously in control of their emotions and cannot simply summon emotive inspiration on command. One could, however, coax or entice the desired emotions.
At first Stanislavsky included what actors call Emotional Memory (based on French psychologist Theodule Ribot’s concept of ‘Affective Memory’) wherein an actor utilizes memories of his own past experiences to emulate the emotional life of his character. Emotional Memory became a controversial method after Michael Chekhov, one of Stanislavski’s protégés, suffered a nervous breakdown. Although Stanislavski shied away from Emotional Memory exercises in his later years, they remain a common (if somewhat controversial) staple of actor training in the Western World. Later, The System was expanded to include the study of physical actions, gesture, verbal/physical communication, and the nature of the Given Circumstances to evoke the actor’s own natural emotions for a performance on stage.
Stanislavski believed that if an actor completed all of the necessary steps of The System, the desired emotional state would be produced within the actor and the audience would perceive his/her actions as genuine and truthful.
These ideas began to migrate to the United States in the 1920s where they flourished and were further developed and adapted by American artists like Lee Strasberg, Sanford Meisner, and Stella Adler.
Actors employing these methods began to appear in major screen roles in the late 1930s, but the new, naturalistic style really began to take over in the 1950s-1960s after the success of films like A Streetcar Named Desire. Today, The System and its countless derivatives are so common in actor training that they have almost replaced the older, more representational acting styles completely.”
Stella Adler image and quote from http://quotespictures.net/stella-adler-art-reminds-you/