"Preparation" and Destabilization
Emotional preparation is the process of creating in the actor (as-the-soon-to-be-character) a condition of extreme or dramatic emotional destabilization. That condition will enable the actor to subsequently create (when the actor enacts the scene) character actions at compelling emotional levels: a person/character grasping for a branch (let's say, the action in a scene) will be more compelling when they emotionally are about to slide off a cliff into a 3000-foot crevice as opposed to a character reaching for a branch to forstall a less emotionally consequential 3-foot fall onto a soft mattress.
This actor's state of pre-scene emotional destabilzation or imbalance is generally attained by various methodologies, traditional (SEE Stanislavski and other 'Method' theorists) and/or a whole range of personal individual techniques, exercises and approaches (including, by the way, the emotional generating act of intellectual analysis of the scene) to create within the preparing actor an exciting and dramatic destabilized (needy?) emotional condition prior to entering the scene. (That emotional need will be subsequently stimulated in the scene by the actor-as-character seeking the character's objective or goal in the scene. Emotional preparation therefore requires an actor create a state of emotional imbalance in himself/herself prior to operating as the actor-as-character in the scene, which will manifest itself in the seeking of subsequent emotional balance through plot or action attainment.)
A few simple examples: an actor prepares her capability for requisite deep loneliness
prior to entering a scene seeking a date at a singles bar; the actor prepares her capacity for rage prior to entering a scene calling for her (as character) to attack and destroy an enemy compound; the actor prepares his or her capacity for happiness prior to entering a scene as character-as-father or character-as-mother scripted to read happily bedtime stories to his or her daughter.
When any of these--or any other--scenes are played out by a properly prepared actor, the actors will emotionally operate in the scene (as 'characters'; seeking the scenes objectives) in an unconscious and automatic manner that will inevitably manifest and reveal externally the actor's previously prepared emotions, and at a level of actor-as-character intensity, complexity and variety that will enhance his/her scene performance.