Judd Apatow can do no wrong, apparently. Even in taking on a genre parody, an endeavor that would seem painfully hackneyed by now following “Scary Movie,” “Epic Movie” and the like, the comic mastermind behind “Knocked Up” and “Superbad” manages to find fresh laughs again and again.

“Walk Hard: The Dewey Cox Story,” a takeoff on the music biopic, hits all the familiar conventions we’ve seen in overly earnest movies such as “Ray,” “Walk the Line” and even “La Vie en Rose,” with its ballyhooed portrayal of Edith Piaf by Marion Cotillard. (This time, longtime Apatow friend and collaborator Jake Kasdan directs, and the two co-wrote the script.) The marginally talented country rocker Dewey Cox (John C.

Reilly) comes from humble, Southern beginnings and experiences tragedy early – his brother’s death, which haunts him long into adulthood. He marries his childhood sweetheart and eventually fulfills his dreams of music stardom through sheer heart and grit, even though the ones closest to him never quite believed in him.

Along the way, of course, he gets hooked on and quits every drug imaginable, has countless wives and children, and hangs out with legends such as the Beatles, Buddy Holly and Elvis. (The cameos for these roles are classic, and the less you know about them going in, the better.) Apatow and Kasdan sometimes don’t know when to let their jokes die a graceful death, beating several of them into the ground.