She famously cropped her hair to play a girl impersonating a boy in Boys Don't Cry. Now Hilary Swank is preparing to cut off her ponytail-length hair once again—only this time, it's for her role as spokeswoman for Pantene's Beautiful Lengths campaign, which encourages people to chop their locks for cancer wigs. "Statistics show that one of the reasons women don't want cancer treatment is because they don't want to lose their hair," she says. "I thought it would be wonderful to give a woman a real-hair wig, which may help her feel like she's going through the process with dignity."

Even so, Swank admits that lopping eight inches off her head (which she plans to do this fall) won't be easy. "I finally got it long again!" she sighs. But on the set of her July 18 shoot for the campaign's public service announcement, as Swank, 32, joined PEOPLE's Jennifer Garcia to pore over pictures of her physical transformations through the years, she got excited about her latest change. "I like switching it up," she says, "doing things that are sort of unexpected."

1992 AT THE MALL
"Oh my, that is hilarious! My two girlfriends Heather and Kristen all went to the mall to get a shot taken. I'm curious to know why I'm wearing white and they're wearing red—maybe we could do a new Charlie's Angels ad! My hair is the same color [as it is now]; that's my natural color. I wanted height in my bangs—we all did!"

1994 THE NEXT KARATE KID
"Wahooo! That was the poster for The Next Karate Kid. I had some flexibility! I remember doing that shot, and when I was done, I was so sore I couldn't sit down."

2000 BOYS DON'T CRY
"Oh, I really cut my hair for that! I remember feeling so excited to be part of such an incredible movie, I wasn't even nervous about my hair. But when I went to the Astor Place Barber Shop [in N.Y.C.], they wouldn't cut it—I had a big ponytail, about 12 inches of length, and the woman just wouldn't do it!"

2004 MILLION DOLLAR BABY
"It was a real physical transformation, [adding] 19 lbs. of muscle—just look at my arm! Training was five hours a day, six days a week."

2005 OSCARS
"I had worked so hard [on my body] that I just thought, my back is never going to look like this again, I better take this opportunity. That was the 'Show it now' look!"

2006 THE BLACK DAHLIA
"It was so much fun to put all those clothes and makeup on and play a femme fatale. I certainly don't get to do that every day—that would be a lot of work, wouldn't it, to do that every day?"

2007 ISCHIA, ITALY
"It's a different body. I don't have 19 lbs. of muscle in that photo—and I certainly don't work out five hours a day, that's for sure!"