Howard had recently given birth to son Theo, now 2, when McG, the director of Terminator Salvation, offered her the role of Kate, the wife of the film's hero John Connor, played by Christian Bale.
"I had a pregnant body with no child in it," she told PEOPLE at the movie's Los Angeles premiere Thursday.
Howard, 28, says she was offered the part over the phone, so wasn't sure how to tell her director about her post-baby figure. "I was so excited, but then was just kind of waiting to tell him, 'I'm not in great shape right now,' " she says. But when she did break the news to McG, "He said, 'Oh, that's okay. [Kate's] really pregnant. That'll be even better.' "
Cookies on the SetWhile the actress – the daughter of director Ron Howard and wife of Dirty Sexy Money star Seth Gabel – didn't have any action scenes, her body did begin to slim down on set – prompting her director to take desperate measures.
"Naturally just after a woman has a baby she starts to lose weight," says Howard. "McG was throwing cookies at me the whole time, 'You're still eight months pregnant [in the film]! Be careful! Continuity!' "
Today, Howard is back to her usual proportions thanks to "good old-fashioned diet and exercise, and homeopathy to help me get my hormones back in shape, because that goes crazy after you have a child," she says.
Still, she wasn't taking any chances at the Terminator Salvation premiere. Her snug, multicolored Mario Schwab dress showed off all her curves. "I felt like it was Spanx," she says with a laugh. "I have a 2-year-old and I need something to just suck it all in!"