The director says, "Action," and you gotta pull something out from somewhere because that big lens of truth staring at you sees it all. What people don't realize is that the second it's quiet on set and someone yells, "Rolling!" everything you planned to do goes out the window. Everything you thought you wanted to express physically disappears. If you think too hard about it, all those 35 extra people watching will knock you right out of your head, and then you might as well just be naked and vulnerable right there in the middle of the room. Scrawny, pale body blinding everyone under all those hot, white lights.
Even after you have 20 minutes to prepare, to get in the mindset you want, the second you heard, "Action!" you whole body tenses up and you heart is in your ears. You forget everything. Having never acted for real before, that would make sense.
Once things get rolling and you're more comfortable, you start thinking of everything under the sun to get your emotions rolling. After a while, you could be all alone. Maybe, you'll get to the point of no return, swallowing that lump in your throat down and trying to keep it there. Forcibly staying in that mindset, by the time it's over, if you've done your job well enough, you might be shaking. Either from adrenaline or something else. And, "Cut!"
After you've calmed down, you have to do the same thing all over again.
After it's over, you feel like you can function again. Like you've been holding your breath for hours and finally are sucking in big gulps of air. You feel better than you have in a long time. Energized, even.
If there's one thing to be learned from this experience it's that sometimes all the preparation in the world sometimes doesn't help. Sometimes you just have to hold on for the ride and let the wind take you.