Reese Witherspoon marked 15 years of her hit movie Walk the Line by revealing it was one of the most "rewarding experiences" of her life.

The 44-year-old actress took to Instagram on Wednesday to share a snap of herself and co-star Joaquin Phoenix dressed as their characters, country music legends June Carter Cash and Johnny Cash.

"Wow! Today marks 15 years since the release of #WalktheLine," she captioned the shot. "Playing the role of June Carter was one of the most rewarding experiences of my lifetime.

"From the stunning costumes created by @ariannephillips to recording all of those classic country songs with T. Bone Burnett, to the incredible scenes written and directed by James Mangold, I felt completely transformed into an authentic country artist."

Reese won an Oscar for her role in the 2005 flick and, continuing her post, she went on to share one of her most memorable moments from its production.

"I will never forget the proposal scene... how Joaquin looked with the light beaming behind him asking June to marry him in front of a full audience. You could hear a pin drop," she recalled, before sharing her appreciation for those who worked on the biopic.

"Thank you to the entire cast and crew for bringing Johnny & June's love story to life," penned the Little Fires Everywhere actress.