The story of Dewey Cox, whose life spins out of control, was written for Reilly who most recently sang on-screen as Roxy Hart's husband in the musical "Chicago." He has also sung and played harmonica in blues bands.
Judd Apatow and Jake Kasdan will write and produce, and Kasdan will direct, Daily Variety reported.
"It's an idea Jake had after seeing one too many musical biopics and feeling it was time to have some fun with that," Reilly said. "My character is an amalgamation of a number of classic musician stories, tales of excess, highs and lows and bad behavior."
Production is expected to begin early in 2007.