The 36-year-old actress confirmed in an interview with E! News on Monday that she will portray the '60s music icon in the forthcoming movie Janis.
"As far as I know, [it's happening]," Williams said. "I figure there's one good day in the life of an actor, which is when you get the job. Before you get the job, you're just like, 'Come on! I'm begging. I'm dying for this. I gotta have it!'"
"You get the job, you're like, 'I'm so excited. I can't believe it. I can't believe I have this opportunity. I can't believe I'm gonna get to do this,' and then you start worrying the next day. So, I'm in the worrying the next day [phase]," she shared.
The Guardian had reported in October that Williams was in talks for the role. The movie is based on the book Love, Janis, written by Joplin's sister, Laura Joplin, and will be directed by Sean Durkin (Martha Marcy May Marlene).
Joplin came to fame as the lead singer of Big Brother and the Holding Company and released her first solo album, I Got Dem Ol' Kozmic Blues Again Mama!, in 1969. She died of an overdose at age 27 the next year.
Williams is known for playing Jen Lindley on Dawson's Creek, and has since appeared in Brokeback Mountain, Blue Valentine and Shutter Island. She will next star with Casey Affleck in Manchester by the Sea, which opens in theaters Friday.