Many actors when memorizing lines for a play tend to rehearse in their heads. Most people are taught that way as children. Parents and teachers would like the children under their care to be quiet. Well-behaved kids are quiet. Bad kids are loud. But the whole basis of theatre is sound. An audience is a group of listeners and the actor's job is to convey his part to an audience with the use of the sound from his voice. That sound has to be practiced every day. That sound has to become part of the actor's everyday experience.
An actor has to practice his or her sound with the neck and throat muscles, with the lung muscles, the abdomen and the face.
Why you need to rehearse your lines out loud. Number one rule of rehearsing lines is to rehearse them out loud. Don't rehearse your lines in your head. Your mind can play tricks on you and give you the illusion that you know your lines when you read them and learn them silently. When you rehearse them out loud, you know when you have your lines or not. When you rehearse your lines out loud it also gives you a chance to play with the lines, to try different approaches. Rehearsing out loud gives you a chance to practice your breathing, to support your lines with your breath. It gives you a chance to practice being spontaneous. Rehearsing your lines out loud may make people in the next room think you are crazy, but if you're going to be an actor, now is not the time to get all shy. Rehearsing your lines out loud is a great way of memorizing because it becomes muscle memory. An actor rehearsing his or her lines out loud is like an athlete practising for the big game, the big match. It is training your muscles and your brain to work together to achieve a goal, and in this case, it is to learn the lines.
How do you rehearse out loud? Begin slowly, taking your time with the lines. Speak slower than you normally would, practising the consonants and the breathing. Play with the sound of the words and the meaning. Identify where the different thoughts come in, the major ones, and the minor ones. Rehearse the lines out loud until they become part of you, until you connect with them. When an actor is not connected with the lines, when he or she hasn't rehearsed enough, the breathing is wrong, the meaning is lost. The actor and the character are two distinct people. Rehearsing out loud helps you to merge the two, so they become one. If you rehearse in your head, you can only accidentally give a good performance. Rehearse out loud and don't give yourself an excuse to fail. Just as you cannot learn a song in your head, or train to run a race in your head, you equally cannot learn to act in your head. Acting is much more athletics than academics.