LOS ANGELES—There’s no disputing this about Joanna Newsom: she certainly has a distinctive voice. For every rave review her music invites on Pitchfork, there’s an army of Internet trolls who prefer to bash her unorthodox pitch. A critical darling in the music world since her first LP, 2004’s The Milk-Eyed Mender, Newsom also has a legion of fans who love what she does. As it turns out, one of those fans is director Paul Thomas Anderson.

At a conference in a media downtown L.A. hotel, the first-time actor seems nervous. While Andy Samberg’s spouse certainly has on-stage experience—including Letterman and the Royal Concert Hall in Glasgow—she’s only ever had to play herself.

In Anderson’s Inherent Vice (opening January 9), adapted from Thomas Pynchon’s novel of the same name, Newsom takes on the role of the all-knowing character Sortilège, who also narrates the adventures of private eye/recreational marijuana smoker Larry “Doc” Sportello (Joaquin Phoenix). It was Newsom’s voice that led to the role.

“Paul is a friend of mine and we had discussed our mutual love of the book in the past, and I think he was experimenting or toying with the idea of having a narrator,” Newsom says. “And he texted me at one point and asked if I would help him try this idea out, basically. So he would email me fragments of narration, which were mostly passages from the book, and ask me to record them into my phone and send them back so he could get a sense of what it would sound like if read by my voice. And then I think once we did some of those back and forth for a while, he decided it was an idea he wanted to pursue for the film.”

Though she has numerous scenes in the movie, Newsom didn’t know an on-screen appearance was in the cards until the production started to get going. “I didn’t audition, nor did he [Anderson] mention it at any point,” she explains. “The costume department at Warner Brothers called me for my measurements at one point and I was just, like, ‘Okay, I guess that’s on the table in some form.’ ”

The voice we hear in the movie isn’t the one we know from Newsom’s records. She’s calm and in control, evoking a hippie vibe that works well in the ’70s-set film. It’s in direct contrast to what those aforementioned trolls like to categorize as a combination of a cat and a witch. More than anything, her presence in Inherent Vice gives the illusion that she’s perfectly relaxed on the big screen—which definitely was acting.

“It was as comfortable as I can imagine,” Newsom admits. “I was still pretty nervous because it was my first film and I didn’t know what I was doing, but anything that anyone else could control made me feel really comfortable. No one made me feel like I didn’t know what I was doing, but I definitely didn’t know what I was doing.”