knowledge of persona

"With the stand-up comic on TV, whether it's Seinfeld or Cosby or Roseanne, more important than their knowledge of how to tell a joke is their knowledge of themselves, or the persona they've created as themselves. So that when you're in a room with writers you can say, 'Guys, that's a funny line but I wouldn't say it.' "
Bob Newhart, New York Times, 26 March 2006

How many of us know our persona well enough to know what we would not say?

And how do we learn a new persona when we have little idea how others see us, often because that vision is so variable?