I've mentioned before such things as the suspension of disbelief, which if you're in the film or TV industry, you'll hear from time to time, or maybe on the Congressional finance committee. The audience has to set aside the reality of the stage, the lighting, the seats and the fact that they're in a room with two hundred other people, so they can get into the story.
That process starts with the set: little details like period-correct fashions, hats on the men, no quartz watches or other anachronistic elements that jar. I can't remember the movie but I think it was The Ten Commandments; if you're on the ball you can spot a wristwatch on Charlton Heston. As fun as that sort of thing is to spot when you're a film nerd, it pulls the viewer out of the thrill of the story itself, which is why he's there in the first place.
The next bit is the players. How right are they as their characters? Do they act is if they're acting, or do they act as if they're reacting?
Let me clarify: I can say a line because that's the part that comes next in the script, and that's acting. Or I can say a line because that's what my character would do - that it happens to be in the script is coincidental.
So far one of my favorite exercises in this process has been giving my character a back story. I created a history for him, one that explains how he got to where he is today. This took a few hours and a fair bit of research. It explained why he didn't have a distinguished military career (not that he shirked the duty, mind - he's a home front soldier, supplying the boys at the front) and went so far as to describe how he came to be in the predicament he's in now, stranded on the island with a bunch of strangers who may or may not be bad people.
Sounds a bit like "Lost." Maybe I should've watched that show from time to time.
So I surprised even myself when I stepped onstage to deliver the lines after a tragedy has befallen
It didn't help any when the director said, "That's amazing. Can you come back in and do that again?"
Blubbering helplessly, "It took me fifteen minutes to get into this state!"
"Well, do your best."
So I came back in and did it again. And it's so draining, so exhausting, I understand why actors don't do this every minute of every day. Acting is hard work. Worth $20M per picture, probably not - anybody can do this. But is it a real job, you better believe it.