To those moviegoers who’ve expressed some weariness of Will Ferrell and Jack Black always doing what they do: Well, OK, here’s somebody else doing it. Rainn Wilson plays the schlubby erstwhile drummer for a leopard-skin-and-power-chord concern now very pointedly on top of the world without him. But his teen nephew Matt (Josh Gad) has a band, too, and can you say comeback? No? That’s fine. You don’t have to say it. Screenwriters Wallace Wolodarsky and Maya Forbes (working from Ryan Jaffe’s story) have solid TV-comedy credentials, and director Peter Cattaneo at least knew how to enliven situational clowning in The Full Monty. But in The Rocker, they all seem to assume that the audience knows the drill anyway, so why bother with any details beyond the simple framework for Wilson’s funny minutiae? Yeah … uh … good question. The band kids do all right with what they’re dealt, as does Christina Applegate as one player’s wised-up mom.