Her 33rd birthday was almost over -- but Jennifer Lopez had one more surprise coming. As the clock ticked toward midnight on July 24, the actress left her trailer and strolled to the steps of New York City's Metropolitan Museum of Art, ready to run through a scene for the romantic comedy The Chambermaid. Instead she found the cast and crew of 200 gathered on the steps, serenading her with "Happy Birthday to You." "I love you guys!" yelled Lopez. She then grabbed two slices of birthday cake and trotted back to her trailer. J.Lo, closet glutton? Not exactly. Ben Affleck was waiting inside -- and even a guy dating a sex symbol has gotta eat.

Those who raise their eyebrows at the two stars taking their romance public before the ink dries on Lopez's divorce papers? Let them eat cake too. "When she wants to do something, she doesn't wait," says Lopez's friend Gilles Bensimon, a fashion photographer. "She wants it now." On July 22, three days before she and her second husband, dancer Cris Judd, 32, filed for divorce, she was already introducing Affleck, 29, as "my new boyfriend" to pals over dinner at the chic Manhattan restaurant Nobu. "She couldn't move forward when things were unresolved," says a source close to Lopez. "She likes everything clear."

And for the moment at least, everything Ben. "She thinks he's really smart and really funny," says her friend. The Bronx-born Lopez befriended the Boston-bred Affleck on the set of their crime comedy Gigli last winter. In March Affleck took out an ad in the trade paper The Hollywood Reporter lauding Lopez's "graciousness of spirit, beauty in courage, great empathy, astonishing talent, real poise and true grace." Lopez was just as taken with Affleck. "We both really kind of surprised each other," she told Oprah Winfrey in May. "I just thought, you know, he was one of these Hollywood guys, (but) he was just, like, a really down-to-earth, charming, sweet, affectionate person."

Rumors of an on-set fling were later denied by both. So were reports of an early summer hookup. But Affleck -- who joined Lopez at the opening of her restaurant Madre's in April and dined with her July 13 at another L.A. eatery -- was clearly smitten, says one source: "He's been following her around a little." Both sides insist things didn't turn romantic until Sunday, July 21 -- a few days after Lopez and Judd agreed to terms to end their 10-month marriage -- when Affleck flew from L.A. to see Lopez at her $75,000-a-weekend rental spread in the Hamptons town of Wainscott, where they were seen kissing on the beach.