I first became aware of this phenomenon in high school, when one of the English teachers cast and directed the only play I've ever been in (although I'd already had lots of stage experience as a musician), Anastasia.
The young lady the director chose to play the lead, I regret to say, was an utter non-entity of whom none of my friends or I (outcasts ourselves in our own way) had even been aware. You might say she was an ultra-wallflower, rather like the invisible girl in that episode of Buffy the Vampire Slayer you may remember. And yet she was so utterly brilliant and appealing in the difficult role that she brought tears to everybody's eyes, and she earned a long, well-deserved standing ovation.
I have no idea what happened to her afterward.
There are exceptions, but in general, actors are people so empty, so devoid of personality, they need others to fill them up, writers to put words in their mouths, directors to tell them which piece of tape to stand on, when to move and how, specialists to dress them and apply paint to their faces, and a horde of other creatures exactly like them to inform them—through a sort of neural network like the nervous system of a jellyfish—what they should think and say on their own time.