George Clooney: Before He Was Famous

Clooney Becomes 'Sexiest Man Alive'

"He was then on the cover of Newsweek magazine and the world turned upside down," his dad recalled.

"George Clooney has proven that he's more than just a matinee star on screen," said O'Neill. "He's worked behind the screen as a writer, as a director, and now he's giving these kind of full bodied emotional performances that show us he's the real deal."

Looking back on his son's early years, Nick Clooney sensed that his son's perseverance would make him a star.

"Most of us, whatever we're doing, we find a plateau. It is comfortable. We sort of stay there, kind of hone it, which George didn't do," he said. "He didn't like to be comfortable. He liked to challenge himself. He wanted to get to the next step. So being funny is good. But being good is even better, and he got better and better at everything and he continues to do that now."

Watch "Before They Were Famous," on a special edition of "20/20" tonight at 10 p.m. ET