By Marisa Wong
August 15, 2005 12:00 PM

The signal that Jessica Simpson, after two months of dieting and sweat-dripping workouts, had finally achieved a derriere fit to don those famously short Daisy Dukes? Husband Nick Lachey—who was at Simpson’s side for a month and a half last winter while she filmed her new movie The Dukes of Hazzard in Baton Rouge—”came by and gave me a pat,” she says. “He said, ‘Your butt looks good!'”

That sentiment can be applied to the rest of her body as well. In preparation for her screen debut as the scantily clad Daisy in Dukes (for a review, see page 31), Simpson, 25, spent weeks in the weight room resculpting her physique. “When I found out I got the role, I went straight to the gym,” says Simpson, who hired trainer Michael Alexander, a longtime friend, to work with her six days a week (see box) for a month before the shoot and three to four times a week during filming. “Any character that has a pair of shorts named after her, you’ve gotta at least try to make the shorts look as good as you possibly can.” Plus “she realized her butt was going to be plastered on 50-ft. screens everywhere,” says Alexander. “That motivated her.”

Alexander put Simpson on a workout regimen of running combined with squats, lunges and weight-resistance exercises to “lean her out,” he says. “She was strong, but I realized her endurance wasn’t that great.” Exercising nearly two hours a day, Simpson felt the results. “I didn’t have a butt, so I did lots of squats,” she says. By the time she was in shape for shooting, “I was like, ‘You can make the shorts a little bit shorter,’ ” she says.

The actress also said no to favorites like bread and banana pudding and adopted what Alexander calls the “Don’t Eat Crap Diet”: a South Beach Diet-style low-carb, high-protein menu of mostly grilled chicken, fish and green vegetables. If she ever cheated—”I can’t deprive myself of things because then I obsess about it and end up eating,” she says—she would make it up with an extra-intense workout. “I watched my diet and was in the gym to walk proud,” she says. “My body is definitely an accomplishment.”

So is her acting debut, say costars Seann William Scott and Johnny Knoxville. “She had a lot of charm and a great sense of humor,” says Scott. Simpson turned down other movie offers but signed on to portray actress Catherine Bach’s original character because “I really wanted to hold out for the perfect role,” she says. “I always looked up to Daisy and thought she was a strong, smart woman.” On the set “they brought in [an acting] coach to help her,” says Knoxville. “The guy was like, ‘She’s on point; here’s your money back. I don’t need to do any work.'”

With Dukes‘ filming behind her, Simpson has been able to relax a bit on her fitness regimen. She now keeps workouts to three times a week and is less stringent about sticking to her diet. (On a July 31 publicity trip to Toronto, she even indulged in a plate of pasta for dinner.) Lachey doesn’t mind the slacking. “She’s still in great shape,” he says. “You’ve got to find that middle line between obsessive working out and also enjoying your time. So what we’ve done is increased the amount of sex.” And as to those who question the authenticity of her rump in the movie: “Padding?” she exclaims. “Padded with muscle! That butt was all mine.”

Marisa Wong; Alison Singh Gee; Jessica Herndon in Los Angeles; Katy Hall; Amy Longsdorf in New York City; Heidi Staseson in Toronto