Power
Unless you're Brad or Angelina (or the dreaded "Brangelina" two-headed celebri-monster) or their star-studded brethren, the amount of power you have is pretty much the opposite of all the tears you're going to cry when you don't get a callback (a.k.a. none vs. all of them).
You'll go out for auditions when you're lucky enough to get them, and you'll be on set exactly when the director and/or producer tells you to be. Otherwise, you'll sit. And wait. And hope. And convince yourself that you're really doing what you were put on the planet to do.
You'll have to take practically every part that's offered to you, especially when starting out. You'll have some freedom to interpret the words on the page when performing, but even then you're something of a slave to the writer's "sacred" text.
You're a puppet minus the strings (and job security). If the producer's third wife decides she hates you one day, you're dunzo. But to be fair you probably shouldn't have parked your piece of junk car next to her brand new Mercedes.
Our advice? You (hopefully) will be doing this because you want to express yourself creatively, so express yourself creatively. Write a play, produce a short film with funding from friends, make a YouTube channel where you dub over cats meowing with funny voices.
Just work hard, because eventually, that hard work may just catch the eye of people who have power. And if they like you enough, just maybe they'll give you some.