It's a work ethic born of a blue-collar upbringing in West Philadelphia, where dad Willard ran a refrigeration company and mom Caroline was a member of the school board. He considered applying to MIT for a career in engineering before opting for entertainment.
But he credits his first girlfriend for the drive for fame. After learning she cheated on him, Smith vowed he'd never again let someone make him feel inadequate.
"When I was doing Ali, I realized that he kept saying, 'I'm the greatest, I'm pretty,' to make himself believe it," Smith says. "He doesn't believe it, but he was dealing with racism. He was reacting to pain and rejection. He said it so much that he started to believe it. That's what I've tried to do for myself."
Hancock co-star Charlize Theron considers Smith's spell on fans almost cult-like. "He's making them drink the Kool-Aid," she says. But, she's quick to add, "He really is that guy. He really is that energetic. He really loves people. He loves life. He doesn't take it for granted. He goes through life like a steamroller."
Sometimes, a headstrong steamroller.
"He has thought out every scene, every word in the screenplay, and he has a theory about all of them," says Hancock director Peter Berg. "He's scary smart. He plays chess. He taught us all how to solve the Rubik's Cube (a skill he learned for Happyness), though he's still the only guy who can do it."
And Smith stands his ground if he thinks a scene doesn't add up, either for laughs for action.
"It's rare that a decision is made in Will's world until all identifiable options have been discussed," Berg says. "He can mentally exhaust you. There were times I had to say, 'Will, I'm going home. I don't have the strength to battle you anymore.' But his instincts are usually right."
Correction, Smith says. It's not all instinct.
"I think of the universe as this big, master computer," he says. "The keyboard is inside each of us. I have a keyboard inside of me. I just have to figure out what to type, learn the code, to make the things happen that I want."
Contributing: Donna Freydkin