Based on the inspirational life story of teen surfing sensation Bethany Hamilton, whose left arm was bitten off by a tiger shark in October 2003, the movie features Underwood as Sarah Hill, a youth counselor at Hamilton's church.
"Sarah has been my best friend since I was very young," Hamilton, 19, tells PEOPLE. "After my shark attack, she always knew what to say to push me on."
Filming for the movie – whose cast includes Anna Sophia Robb as Hamilton, and Dennis Quaid and Helen Hunt as her parents – began earlier this week on Oahu's North Shore. While she takes a break from her studies at Brown University, Jack Nicholson’s daughter, Lorraine, will portray Hamilton's best friend, Alana Blanchard, while Kevin Sorbo plays her father, Holt.
Though Underwood, 26, has reportedly turned down various movie roles in the past, she fell in love with Hamilton's story after reading the Soul Surfer script. She'll now spend most of next week filming on Oahu, before hitting the road and returning to Nashville to prepare for her upcoming concert tour.
"I can't wait to see her on the set," says Hamilton, who has gone on to become a top-performing pro surfer despite her injuries.