Juliette Lewis is in the final stages of prepping a feature documentary about her musical career and is aiming to bring the finished film to Sundance in January.

The 38-year-old thesp has racked up extensive feature credits including “Cape Fear,” “Natural Born Killers,” “What’s Eating Gilbert Grape,” “Conviction” and “Due Date.” But she also embarked a decade on a second career as a rock singer.

Lewis partnered with Chris Maybach and Patrick Cunningham of Maybach Entertainment, producer of “Martha Marcy May Marlene,” and her manager Brandy Lewis to produce the doc. Saerom Kim and Saemi Kim are exec producing.

“I first heard her sing four years ago when I was at Cannes and I was blown away at how good she is,” Cunningham said in an interview this week following the premiere of “Martha Marcy” at the Toronto Film Festival.

He and his producing partners have culled about 400 hours of footage into a nearly finished cut of the yet-to-be-titled pic.

Lewis’ career has covered hundreds of gigs, first with Juliette and the Licks and then with the New Romantiques, with whom she recorded the album “Terra Incognita.” She’s performed at festivals in Leeds, Redding, Brazil and on the Warped Tour.

“I knew when I was 28 that I had to keep pushing myself into areas where I felt uncomfortable and exposed,” Lewis said in an interview in Toronto this week. “I’ve always been interested in living dangerously so going into music like this has sort of been my unleashing my love affair with my audiences — a joyful middle finger to the world. I have a real muscular voice that gets compared to AC/DC.”

Lewis stressed that the film won’t be a vanity project, but rather an exploration of how performing live as a singer became a full-time primary career with extensive social networking. “In music, your fan base is everything,” she adds.

Lewis is also a huge fan of “Martha Marcy,” which premiered at Sundance, with Sean Durkin winning the festival’s U.S. directing award for best drama, and also screened in Un Certain Regard at Cannes.

“It’s the best film I’ve seen in 10 years,” she declared.

Lewis joined the NBC mid-season drama “The Firm” in July.