Jada Pinkett-Smith, 29, faced a fitness challenge not shared by Keanu Reeves and company while prepping for her strenuous role in the upcoming

Matrix sequels: The actress recently gave birth to her second child — daughter Willow Camille Reign Smith — with hubby Will Smith.

"I'm still breast-feeding, which made it a little harder to get in shape," she tells TV Guide Online with a chuckle. "But I was ready to lose some weight, so it was great to get physical. I'm getting pretty good at kung fu."

Considering the level of secrecy surrounding the two new Matrix pics — which are being filmed back-to-back — Pinkett-Smith has been forbidden from revealing much about the plot. "I play a kick-ass chick and I'm really digging her," she previews. "She's very tight with Laurence Fishburne, but that's all I can tell you."

Pinkett-Smith — who recently completed a small role opposite her better half in the Muhammad Ali biopic, Ali (in which Will stars as The Greatest) — can next be seen in the family dramedy Kingdom Come (opening Wednesday). "Whoopi Goldberg plays my mother-in-law," she says, "and she really got on me about my comic timing. Whoopi kept saying, 'Let that line live. Don't step on the beat.'"

Goldberg did more than just coach her on line readings, however. "She was like my big sister," recalls Pinkett-Smith, who was pregnant with Willow during the film's shoot. "In one scene, I had to fall on the floor. After one take, Whoopi said, 'That's it. The girl is pregnant, she can't do that anymore.'"

While Pinkett-Smith has been spending a lot of time in front of the camera lately, she isn't letting it stand in the way of her family. "If acting was ever jeopardizing my marriage, I'd quit in a minute," she insists. "I've experienced divorce through Will, who was married once before. It's not a good thing for anybody. We stay close to each other and we stay real by being true to ourselves, not an image of who people think we are."