The first basic assumption about acting is that an actor's body and voice must be trained and flexible so they can respond to all demands. Many actors today utilize this step by taking acting lessons, vocal lessons, and even dance lessons to prepare for a role.
The second basic assumption is to act truthfully, the actor must be a skilled observer of human behavior. Doing things like riding the bus for a while, or sitting in a park in order to watch the many different kinds of people around you and the different behaviors people have are ways for actors to observe human behavior.
The third assumption is that if actors are not merely to play themselves, they must understand a character's's motivations and goals in each scene and in the play as a whole, as well as each character's relationship to all other roles and the dramatic action. This makes it important for an actor to thoroughly read through a script, including other characters parts to fully understand their role. Another way to assist in this is by studying other versions of the performance, if there are any, to see the performance as a whole and how it all fits together.
Stanislavsky's next basic assumption is that actors must project themselves into the world of the play and may learn to do so through the magic if (through imagining how one would feel or act if one were in this specific character in this specific situation). Actors best do this by becoming one with a character, studying the features, feelings, and...