The "magic if" is a tool invented by Stanislavski, the father of acting craft, is to help an actor make appropriate choices. Essentially, the "magic if" refers to the answer to the question, "What would I do if I were this character in this situation?" Note that the question is not "What would I do if I were in this situation?" What you would do may be very different from what the character would do. Your job, based on your analysis of the script, the scene, and the given circumstances regarding the who of your character, is to decide what he or she would do.