How does an American actor/actress come to prominence?

While I can kind of mentally track the careers of a lot of (maybe even most) actors, there are many that come to mind who seem to have just been spontaneously deemed worthy of being in Hollywood movies. What are the stepping stones towards Hollywood? Many people in Hollywood don't seem to very good at acting and if stripped of their monumental budgets, their skills would be revealed even more. And have you seen some of the movies these people are in? Is the joke on us? Are movies bad because they know we will swallow them?