The more I think about it, the more I am convinced that an aspiring actor needs considerable charisma--that mysterious charm and magnetism that audiences recognize in the actor and makes them identify with them. Charisma means that the audience members fall in love with the actor, at least to the point of being charmed by the actor and admiring him or her. Actors need to be attractive to audiences if they are going to succeed to any extent. Audiences want their actors to have qualities that they lack. They want to see happiness and success in actors. They want personality--charm and wit and sparkle, sexual energy, and attractiveness. This does not have to be physical beauty, it could be power, wealth, social position or self assurance.
So what does the aspiring actor have to do to achieve charisma? They can practice being suave. They can learn social graces and practice manners and tact. They also need to practice performing with energy in their demeanor and in their responses--vocal and physical. And they need to learn to love the camera. Whenever they audition, they need to be thrilled to be there, and they need to exude the charm of every character they read for. They need to have the kind of charm like when someone walk into a room, and everyone stops what they are doing and watches them, because they exude the charisma of a James Bond or a Clint Eastwood or a Meryl Streep. Aspiring actors need to work on it. And I suggest the aspiring actor takes plenty of time, doesn't rush, relaxes and that they REACT! Remember James Cagney's advice, "...look the other guy in they eye and speak the truth."
Additionally, you cannot act if you are carrying baggage from home into the scene room. You must have no problems at class, in rehearsal or in performance, You have left all of that home or certainly outside the performance space. You create charisma by enjoying what you do. It shows in your energy and charm, your relaxed and happy demeanor of loving being there and having no other care in the world.
Otherwise, the only prep you need to do is to create the physical character and learn the lines. Then don't think when you act, just listen and react. There is no half way to knowing your lines, you either know them or you don't. It is the actor's responsibility to know his lines. It is impossible to act without having the lines down to perfection. Knowing your lines frees your mind to receive the stimuli of the scene and to respond honestly, fully and without inhibition. It relaxes you the same way that meditation or yoga relaxes many actors. Being relaxed clears your mind for being charming and frees the body to respond with energy. There is no room in an actor's read for anything but the scene. That is why Mamet includes leaving home at home and the theatre at the theatre in his list in True and False of behavioral traits that actors must have. Actors who have charisma are not preoccupied by anything. They can concentrate on the task at hand.
To do your very best in your audition, your performance and in your career, you need to develop strong charisma. It is the undefinable quality that makes an actor a star!