“When we set out to make the music, we knew the songs had to be funny,” says “Walk Hard” star John C. Reilly. “But we quickly realized they had to be good too.” Reilly spent four months recording Dewey Cox songs, some 40 in all (15 are available on the soundtrack CD, and 15 more on its digital version).

As he notes, “That’s more than some people’s entire discography.” By the time the process was complete, Reilly had the character well in hand, despite the fact that not a frame of film had been shot. “The music was a big part of preparing for the movie,” he says. “Every time we decided on a lyric he would sing, it was saying a lot about him.” “John was living within it,” adds director and co-writer Jake Kasdan.

“When you actually have the guy performing the part, and it is in a way a performance, it makes you start to make decisions about what the guy is like in a broader context than just what’s in the script. You have to answer a lot of questions.” Dewey’s greatest hits were concocted by a stable of songwriters, working closely with Kasdan, Reilly and co-writer Judd Apatow.

Soundtrack producer Michael Andrews oversaw the pitch-perfect evocation of period styles, from ’50s rock through ’60s psychedelia and Dewey’s ill-advised stab at disco. Some songs, like the title track, were written into the script, complete with titles and lyric fragments. Others, like the double entendre-ridden “Let’s Duet,” were conceived in broader terms. And still others came out of brainstorming sessions. “The songwriters really fed on our ideas,” Reilly says.