Begin with accepting the imaginary circumstances of the script or sides. To accept these you need to know the five W questions, the who, what, where, when, and why of this material? Who are you according to what the writers intention is? Who are the other actors in the scene to you and each other? This is all information the writer has written to put you into the scene, to find out what you are doing. The writer in creating the story will give you the information, needed to fulfill his or her vision.
They have created the story with focus and concentration, to the point that every single word was written with a purpose and intent. It is all there if you search for it. Keep asking the five W questions for answers to your questions to put yourself strongly in character for the scene, who are you? What are you doing? Where are you? When is it, and the why of everything?
And then ask yourself what was the moment before this scene started? And what just happened that brought you to this situation? This is all just preparation information that will give you a place to begin.
Of course then you have to forget all this, and express yourself knowing all this is just information facts, and then express yourself truthfully in the moment as to what your experience is moment to moment. The mind doesn’t make a distinction between imagination and reality. Express yourself truthfully of how you feel, not what you think you should say, or what will make the scene work, or what you think the writer has intended, but what you honestly feel.
My first acting teacher, Sanford Meisner, always told us ” You have to be pinched before you say ouch”. You have to have an experience before you speak the words.
Just begin to practice these suggestions, and I believe you will benefit with spontaneous and unpredictable acting that audiences enjoy the most.