John C. Reilly gives voice to 50 years worth of popular music in the upcoming film "Walk Hard: The Dewey Cox," which was written and produced by Judd Apatow and directed by Jake Kasdan. It opens Dec. 21 in U.S. theaters.

Reilly's tour-de-force performance as the Johnny Cash-like Cox was the subject of a panel with Apatow, Kasdan and the film's songwriting team today (Nov. 1) at the Hollywood Reporter/Billboard Film & TV Conference in Los Angeles.

"Before we'd even begun writing, Judd and I almost immediately said it should be John," Kasdan said. "It's a well-known secret that he's an amazing singer and the kind of actor who might play someone for their entire life in a biopic. It's a level of acting that you just wouldn't expect in this kind of movie."

On the Columbia soundtrack, due Dec. 4, Reilly sings all 15 songs, which were penned by such names as Marshall Crenshaw, Van Dyke Parks, Dan Bern and Mike Viola. An expanded edition of the album with 15 additional tracks will be available on iTunes, according to Sony Pictures president of worldwide music Lia Vollack.

"He covers a lot of different genres and a lot of different kinds of singing," Kasdan said. "He was the other major voice in this writing development process of getting the songs together and then making them sound right coming out of his mouth."

Among the songs previewed during the panel was "Let's Duet," which Cox sings in the film with backing vocalist Darlene Madison (Jenna Fischer). "As soon as we heard that phrase, "In my dreams you're blowing me ... some kisses,' we knew we'd found our duet," Kasdan said.

"Walk Hard" features cameos from artists like Pearl Jam's Eddie Vedder, the White Stripes' Jack White and Ghostface Killah, which Apatow said helped "to create a credible world." Apatow even wrote some lyrics with Ghostface, which he describes as "as uncomfortable a moment as you could imagine for me. But he was very cool and was not at all offended when I asked if we could get the word 'shiv' in there."