“If you speak any lines, or do anything, mechanically, without fully realizing who you are, where you came from, why, what you want, where you are going, and what you will do when you get there, you will be acting without imagination. That time, whether it will be short or long, will be unreal, and you will be nothing more than a wound-up machine, an automation.” -Konstantin Stanislavski, An Actor Prepares
To be effective in any scene or entire play, the actor must know (a) what they want, (b) why they want it and (c) what obstacles stand in their way.
Sounds simple enough.
And it is. Provided you do massive prep work beforehand to figure out the answers to these questions. If you don’t?…Well, good luck.
Note: This isn’t just for actors. Anyone looking to accomplish anything would be wise to heed Stanislavski’s advice.