|Wassily Kandinsky - Circles in a Circle, 1923|
When working on a character, it is of extreme importance to ask yourself, "Why am I saying this? Why am I behaving this way?". Your answer should include the causes for a certain action or line; even more importantly, though, it should include its scope, or purpose. It's the latter that allows your character to keep going, projected towards a final goal.
After establishing who the character you're playing is and doing all the necessary work to build the character, there comes building the scene. It is essential for each scene to ask yourself, "What is my objective?", "What am I trying to get here?".
This, together with understanding what you are doing with the other characters in a specific scene, will not only set the tone, but give you a clear idea of what you want to achieve and how you should carry it out.
It is a good rule for your objective to never be a physical one, for example: "My objective for this scene is to slap this person"; if you want to slap somebody, all you need to do is slap them, it's as simple as that. Your objective should be an emotional, abstract one, like, "I want to humiliate this person"; a physical action - like a slap - is simply a way to pull off that objective but never an objective per se.
Also, I don't think an actor should be concerned with consistency.
Of course, if a character is shot in the knee at the beginning of a play, then he's supposed to limp to different degrees for the entire duration of the story. But this is nothing about consistency; this is continuity, another important but different concept.
Emotional consistency is just nonexistent in real life; why should we be limited by it when working on a character? Michael Shurtleff, one of the most famous Broadway casting directors in America, termed consistency as "the death of good acting".
You will soon realize that the script or the play are, for each single character, brought forth by superobjectives. At the same time, scripts and plays are divided into smaller units of objectives according to the number of scenes in them. At a deeper analysis, a scene can also have more than one objective.
Naturally, this is only one of the many aspects an actor should focus on.
If I were a writer, I would use a similar method for my novel writing.
Would that be a mistake?