By now, audiences are well accustomed to the sight of Will Smith running.

In “Bad Boys,” he ran for his life; in “Independence Day,” he ran for liberty. Yet in “The Pursuit of Happyness,” Smith runs to get by, to earn a decent living and to provide a roof over his young son’s head.

Based on the true story of Chris Gardner, a San Francisco father who rose from sleeping in a train station bathroom to becoming a millionaire stockbroker with Dean Witter, “Pursuit” reveals a vulnerable side of Smith auds have never seen before.

“I’ve been very successful with posturing and posing, and I know innately that hero thing that people connect to,” says Smith, who handpicked Italian director Gabriele Muccino to help him reach a more genuine emotional performance. “This is the greatest superhero character I’ve ever played.”

As Smith saw it, the story wasn’t about Gardner’s ultimate financial success, but his noble goal “to try to keep some sanity in his son’s life. I enjoy asking myself, ‘Would I have broken in these moments? Would I have broken in that bathroom?'”

That scene, in which Gardner must find a way to convince young Christopher (winningly played by Smith’s own son Jaden) to spend the night sleeping on the floor of a public restroom, was especially powerful for the actor.

“I’m actually lying on that floor in a bathroom with my real son, so there’s no acting preparation necessary for that,” he says. “It was borderline unbearable. That’s the acting nirvana where you actually believe it for 45 or 50 seconds.”

The real-life connection paid off in less agonizing moments as well, lending a crucial naturalness between father and son to a movie that depends on that dynamic. Smith actually thought of his own working-class father when developing the character’s body language.

“You could always see the weight of the world having physical ramifications on his body, wanting to stand up and wanting to be strong, but that yoke on his neck always had his head leaning forward a little bit,” he explains.

Just as he did for “Ali,” Smith was glad to have the real character on set whenever questions came up.

“There won’t be two shots at the Chris Gardner story,” Smith says. “If I mess it up, he’s not going to have a chance to go get Denzel to do it right. My only goal was for Chris Gardner and his family to appreciate the telling of the story.”

Next project: Currently shooting “I Am Legend,” a “28 Days Later”-like sci-fi movie in which Smith plays the lone survivor after an outbreak transforms all other humans into vampires. After that, he’ll try being a real superhero in “Tonight, He Comes.”