They become gut responses. Ask me to memorise a two-hour long play, and I can do it effortlessly. Ask me to memorise two paragraphs of a History textbook, and I will fail the exam. Prompters became obsolete along with the caveman and the corset. However, the actor is aware of exactly what his character is feeling and thinking at any given point, and can therefore improvise without the audience ever being aware that his knees are knocking.
Rehearsals are periods of excitement and discovery.
The role teases and titilates you, revealing bits and pieces of itself to you as and when it pleases. Slowly, a character starts to come alive within you, a person with different thoughts and emotions, housed within your body. The scripts reveal new twists and turns you never saw before, seducing you into following it on a mysterious journey. During the run of a play, you are priviliged to go through the character's life-experience in full, night after night. You soar with his highs, despair with his downs and revel in a strange mind not your own.