Sometimes the actor is employed to work on a production where there is no time to rehearse his scenes with his director and fellow cast members before performance. This situation is rare in the theatre and more common in films with a small budget. Here, the actor is expected to have the discipline and the skill to rehearse himself, and turn up, on set, ready to deliver the goods. In order to be relaxed and work with confidence, thereby doing his best, the actor must prepare correctly, and further, this preparation will help him to deal with any curve balls that may come his way and avoid freaking out when confronted by the unforseen.
Central to this preparation is identifying and accepting that which is within the actor's control, and that which is not. How the actor feels is out of his control, as is his mood, his confidence. How the actor's colleagues respond to him, their manners and professionalism, or lack thereof, are not under his control and if he understands this, then the actor is less likely to be disturbed by contemptible behaviour. The production itself is not under the actor's control, how well it is going, whether it's on schedule, morale, etcetera, none of this should affect the actor's work for it is not within his control.
But the preparation the actor chooses to do is under his control. In addition to identifying the above, the actor must keep his own house in order, by planning to conduct himself impeccably, by knowing his lines so well that they have become habitual, by choosing a concretely doable action* to play the scene. It's also important to realise that, when rehearsing and performing, emotions are NOT under the control of the actor. Sometimes we come across someone who has a knack for making themselves cry on cue, but this is not a good actor, this is merely somebody with a knack for making themselves cry on cue, and infact, these kind of crocodile tears are the antithesis of the actor's work, which is to perform the scene truthfully, because any emotion which is not created organically by the actor's attempts to do the actions called forth by the script, is a lie. (It is at this point that I implore dramatists to write not emotions but actions).**
Actions are within the actors control, emotions are not. And if our goal on set is to only concern ourselves with that which is in our control, we will forget about emotions and concentrate entirely on our actions. So, the actor arrives on set and settles into position, ready to do the scene. The director shouts ACTION! Now, during his lonely rehearsals, the actor made a choice about how to play the scene, he chose to “demand some peace and quiet” (this is a fictional unwritten scene that I've just made up, but you catch my drift). But, at the end of take one, the director screams: “CUT! No no no! What are you doing, you imbecile! You're too strong! Why play the scene like that?! Lets do it again and this time I want you to be weaker, you need to be frightened of the other actor!” Now, that's quite a radical change the director is asking for. What is the actor to do in this moment of pressure as everyone around him is preparing for the next take, and appears to be doing their job perfectly? Well, he certainly doesn't want to have anything to do with trying to summon fear within himself, or even worse, indicating that he is frightened with his “scared” acting, both options would lead to phoney work. Might the actor simply adjust his action, and instead of “demanding” peace and quiet, he might “plead” for peace and quiet. This requires no special “preparation”, the actor can begin pleading immediately, it's concretely doable, and as such, the actor's work will remain true and full, and of course, the director's demand will be met, unless they want more that is, in which case the actor might add an adverb to his action, and “plead desperately”. So the properly prepared actor then, not only chooses to do an action he thinks will fulfill the intentions of the script best, but he may also come up with reasoned variations on his original choice, and practice doing those too.
Rigorous and detailed preparation will enable the actor to remain creative under pressure, and do his job well, and maintain impeccable standards of behaviour. It is the paradox of acting, that the better prepared the actor is, the freer he is to play.
*An action here refers to the actors attempt to accomplish something. For example: to sell a great idea, to tell off a fool, plead for a second chance, get a straight answer etc.
** If anyone else can come up with more items within and without the actor's control, even if they come from a different discipline such as writing, please do post them in the comments section, as I am always eager to learn. Thanks.