Britney Spears biopic star says the Lifetime movie is 'a feminist story at its core' The cast and director of the unauthorized movie sat down with "Nightline."

 -- Britney Spears, our omnipresent pop idol, had one of the biggest selling singles of all time, "…Baby One More Time." Her debut album, which was released in 1999, was one of the best-selling ever by a teenage artist.

The one time Mickey Mouse Club ingénue went red leather raunchy for the "Oops ... I Did It Again" music video and then donned a bedazzled nude bodysuit for "Toxic." She was also one half of an early '00s pop power couple with then-*NSYNC member Justin Timberlake. Then in 2007, almost 10 years ago to the day, Spears shaved her own head in a California salon, leading many to wonder if the pop princess was headed for a major nervous breakdown.

All of these moments –- and many more -- have been recreated in "Britney Ever After," the unauthorized Lifetime biopic, airing Saturday night.

"[She was] a prisoner of celebrity," said director Leslie Libman. "She can’t go anywhere, she can’t do anything ... it’s like living in a glass box."

Natasha Bassett, a 24-year-old Australian actress, plays Spears and said the movie is not meant to trash the pop star.

"This [movie] is a love letter to Britney," she said. "I think she got into something at a very young age that almost cannibalized her."

In real life, Spears seems to have stability, with a performing residency in Las Vegas and she often post photos on social media of her home life with her two sons. In response to the biopic, a Spears spokesperson told ABC News in a statement that "Britney will not be contributing to the project in any way, shape or form nor does it have her blessing."

The fact that the biopic is unauthorized didn’t bother Libman.

"I didn’t find it as challenging because I didn’t have to stay true to exactly what was going on. It was impressionistic," she said. "An impressionistic telling of a story that everyone knows."

In the pantheon of stars given the Lifetime TV movie treatment, which have included Whitney Houston, Anna Nicole Smith, Marilyn Monroe, Aaliyah, Brittany Murphy, most are deceased. Spears is obviously not.

"It’s always difficult to play someone who’s still with us and can view the work you’re doing," Bassett said. "You just want to do them justice."

Being from Australia, Bassett said she watched Spears' old interviews to master her Louisiana accent.

"I just played her interviews all the time," she said. "If it was a darker time in her life, I would be crying every time I watched an interview."

Bassett admits that she had her doubts at first about taking the part.

"But it was when I read the script I decided, 'No, I have to do this film, I want to tell this story,'" she said. "Because it’s a showing of strength and I think it’s a feminist story at its core."

Amey Rene, the casting director for the movie, said Bassett was "unique."

"So our idea wasn't necessarily to find someone who was a carbon copy of her, it was to find an actor who could embody her essence," Rene said.

The movie centers on Spears' relationship with Justin Timberlake -- the couple dated from 1999 to 2002.

"It's young love, it's like star-crossed lovers. You know, 'Romeo and Juliet,'" said Nathan Keyes, 31, who plays Timberlake. "It's a story we can all relate to."

It also touches on the highs and lows of Spears' career. One of the most memorable real-life lows was Spears’ disastrous 2007 VMAs performance, which was supposed to be her "comeback" performance after two children and two marriages. But the "Britney Ever After" director said she had "no desire to recreate that moment."

"There’s no way I could," Libman said. "I decided that it really was a feeling. And if we could get, have it more visceral and we could feel what she’s feeling then it would be interesting."

One of things that might surprise viewers is the biopic didn’t get permission to use Spears’ tracks for her music, just the rights to the songs she performed, so they have actors singing covers.

"It’s more about who Britney Spears is as a human being, not just a performer," Bassett said.

There have already been mixed reviews of the movie, with some love from the Internet and plenty of hate. But the actors and the movie's director say they aren't fazed.

Libman thinks Spears is "going to love it." Bassett added that she hopes Spears watches. "I hear she’s a fan of Lifetime," she said.

ABC News' Nick Watt contributed to this report