The 41-year-old star's longtime hairstylist Chris McMillan tells W that she was instructed by famed photographer Steven Klein -- who also shot the infamous W cover of Jen's ex Brad Pitt and Angelina Jolie in 2005 -- not to wash her hair.
"At the end of the first day, Steven came up to me and goes, 'Could you please ask her if she could not wash her hair tonight and just show up tomorrow?'" McMillan recalls of the April issue shoot. "I mean, she was rolling in the dirt, it was windy and she had hairspray in her hair."
Still, the Bounty Hunter actress obliged.
"We left her hair dirty," McMillan, who created Aniston's famous Friends-era Rachel cut, says. "It just created a nice, chunky texture. The key to Jennifer's hair is no matter what you do with it -- straight, frizzy, dirty -- it looks like it actually grows out of her head. She's someone for whom her hair doesn't wear her. She wears it."
Overall, her look on set -- the two-day shoot with Gerard Butler took place in Canyon Country, California -- was "not her typical blown-out hairstyle," he says. "It's a little rougher, we liked seeing the flyaways."