Let me remind you that Relaxation and Sense Memory are the foundation and structure of the "house of method". Concentration is the key to the house. The success of any of the other elements of this work depends on a full understanding and mastery of these basic concepts.
RULE OF THUMB: Use these procedures only when you cannot overcome a problem in a scene by naturally inspired means.
Let's say the play demands that you are in love with another person in the story. In life, it is an unfortunate coincidence that you actually have no warm feelings at all for your fellow actor. Maybe you have the opposite feelings.
What do you do? Fake it? No. You could, as one example, look into the eyes of your scene partner, and sensorally create the eyes of someone you actually do love (or the hair, lips, or nose, etc.). If your powers of concentration are strong when doing this, you will forget who you are working with, and magically believe it is whomever you have "substituted" for that person.
Using a substitution to create another person has also been called by some who use this work a "personalization".The reason for this, is because it involves substituting a person, rather than another physical object.
Substitutions can work for literally any situation for which you need them. When I was doing a scene at the Actors Studio-West from " I Never Sang For My Father", by Robert Anderson, I tried an imaginative substitution that helped me deal with the fact that in the story, "Mother" had recently died. In life, my own mother was still alive.
The scene takes place in "Dad's" study. In early rehearsals, I had seen a small cigar box on the stage as part of the set. I decided to use it to help keep me "feeling the loss of Mom".
Looking at the cigar box, I would sensorally create a tiny coffin, and when I would lift the lid, I would sensorally create my mother in the coffin. Sound strange? It worked, and that kind of technique will work for you if you have developed your concentration through the Relaxation and Sense Memory exercises.
Ever find yourself in a play where you have to handle a prop as though it were a rare and expensive antique? Of course, the actual prop you have in your hands cost $1.25 at the local Goodwill store. But if you substitute something from your own life that has personal value for you, the audience will see that the stage prop really means something to you. And you will be relating to that prop in a specific, rather than a general way.
You can use substitutions for any number of situations, choosing any objects necessary to help you create the life you are living on the stage. You can look at the head of John the Baptist on a platter and see a "puppy dog" to give the impression that you are insane. Of course, if you are an actor, you probably are a little insane anyway, huh?