"She can sing," the movie's director, Michael Sucsy, tells E! Online. "And she's going to learn how to play the Dobro [guitar]."
The drama – a departure for Aniston, who normally stars in romantic comedies – is based on a true-life story about a group of women imprisoned in Texas who start a country and western band.
"These women learned to harmonize and they learned to play instruments," Sucsy says. "They weren't these amazing, accomplished singers who banded together. That's what's interesting about it."
"It's not just like, 'Hey, you can play the guitar, I can play the piano, I can play the harmonica. Let's put a band together,'" he goes on. "It's like, 'We have to get out of jail, and if we have to put a band together to do that, then that's what we're going to do.' "
Sucsy plans to cast real-life musicians alongside Aniston, 40, for the band, he says.
Aniston – who is also producing the movie (out next year) – last sang while guest starring on 30 Rock, where she serenaded Alec Baldwin for his birthday.
It's not the first time the director has worked with an actress who isn't exactly known for belting out tunes.
Sucsy directed Grey Gardens, in which Jessica Lange (who stars alongside Drew Barrymore) sings "Tea for Two."
The HBO drama is up for 17 Emmys next month.