A lot of people go into show business to overcome their shyness. It gives them a way to kind of step outside of themselves and express themselves in ways that they can't in real life.
For some celebs who have very extroverted public images, but are actually introverted in real life (like Lady Gaga and Marilyn Manson), their image is kind of who they feel they are on the inside and who they want to be, but are afraid to be in real life, for whatever reason. It's kind of like the movie The Mask with Jim Carrey, when he put on the mask he became everything he wished he could be, but was afraid to be without the mask. Sometimes people can only truly be themselves when they're pretending to be someone else because it's easier to let go of their inhibitions. So, more or less, I guess in some cases, it's the public image that's the act, to hide or overcome how shy they are in real life.