Evangeline Lilly Bleached Her Hair 'Every 10 Days' for New Film: 'By the End It Was Falling Out'

Evangeline Lilly completely committed to her bleached blonde look for her role in the new crime thriller, South of Heaven, that her hair ended up taking quite the toll.

The actress exclusively tells PEOPLE that both she and director Aharon Keshales wanted her character Annie Ray to look like "sunshine and the source of life" throughout the film. "[Keshales] wanted her to be this glowing representation of life," adds the actress, 42.

So becoming a blonde seemed like the best fit. Instead of wearing a wig every day for shooting, Lilly agreed to bleach her dark brunette hair platinum. But it wasn't an easy process.

"I chose for her to have the naturally blonde hair and a kind of warmer complexion so that she would look glowing, healthy, vibrant and full of life," Lilly says. "But that upkeep became really arduous because I wanted her to look like she was a natural blonde."

The star's colorist needed to strike the right balance between giving Lilly a bright (not brassy) blonde look that didn't appear overly artificial.

"I didn't want it to look like a bleached blonde platinum head of hair," Lilly explains. "And my hair is very, very dark naturally."

At the time, Lilly wore her hair in a short, buzzed cut so "it was so short [that] if it grew out by even a millimeter, you could see it," she says. "I was bleaching my hair every 10 days, and by the end of the shoot, it was falling out."

Roy Rochlin/Getty; Evangeline Lilly/Instagram

Before she began shooting South of Heaven, Lilly shaved all her hair off (and did it herself!). While some fans speculated the switch-up was for a movie role, the actress tells PEOPLE that wasn't the case.

"It was interesting because I shaved my head about a year prior to shooting this movie and so I think a lot of people mistakenly thought I shaved my head for this role," she says. "But it had nothing to do with this role."

In fact, she decided to cut off all her hair "just for fun." Lilly adds: "When it came time to do this role, I kind of didn't know if there was anything I needed to do to change how I looked because I already had a shaved head!"

South of Heaven is out now.