Reese Witherspoon is eating a salad for lunch – a sensible mix of greens, light dressing and tuna – and frankly, she's not at all happy about it. "I'm trying not to eat too many potato chips," she says with some degree of longing, her Tennessee accent moving in and out of her conversation like a breeze. "That's all I want to eat. Potato chips and hamburgers. On the first pregnancy I gained a lot of weight. I'm trying to be better this time."

It's early May on the set of Legally Blonde 2: Red, White & Blonde, and Witherspoon, 27, is already four months into her pregnancy. After playing a cat-and-mouse game with the media in April – she tried to keep her pregnancy a secret during the first trimester – Witherspoon is now speaking freely about the blessed event. It was last winter, she says, after she returned from a Caribbean vacation with her husband, actor Ryan Phillippe, 28, and their 3-year-old daughter Ava, that she learned she was once again expecting. It wasn't entirely a surprise. "It's something that I'm always open to," says Witherspoon. "I was just happy if it happened. Ryan's really excited about having another baby. And I'm really excited. And Ava's more excited than both of us. She's dreamy about it. She likes to prepare her room for when her brother or sister comes. I think she'll be a great older sister." For the Legally Blonde 2 scene being shot today – a surprise party that includes costar Jennifer Coolidge – Witherspoon will be seated to hide her pregnancy. During her lunch break, she slips out of her hot-pink chiffon outfit and into a comfy white robe. The belt strains against her round midsection, but the rest of her 5'2" frame remains thin and delicate. "You see, I'm eating salads!" she says, lancing a piece of lettuce with her fork and taking a swig of bottled water. "I'm trying!"

And when Witherspoon tries, she usually succeeds. She's been appearing in movies since she was a teenager, and she long ago gained the respect of critics. Then came Legally Blonde in 2001, in which she starred as Elle Woods, a fashion-crazed but whip-smart sorority girl from Bel-Air who follows a guy to Harvard Law School and ends up at the top of her class. It was followed by last year's Sweet Home Alabama. Witherspoon recalls how "surreal" it was to see her giant head floating above L.A. on the Alabama billboards, though "it was great to get back to my (southern) roots. I think people can tell when you're having fun in a movie." The film's $127 million gross established Witherspoon as Hollywood's most bankable star under 30. "I feel like I'm just finally getting who I am, and it's really nice that at the same time, audiences are kind of getting who I am too," says Witherspoon. And who is she, exactly? She thinks for a moment and answers, "When I had a kid, it clarified a lot of things to me. I'm not a supercool person. And I've never had the most friends. I'm a mom and a wife, and that's what I like to be."