Robert Altman’s A Prairie Home Companion is such a deconstructionist’s dream that you almost expect the late Jacques Derrida to make a cameo appearance. Talk about reality’s insubstantiality: here we have the cinematic version of a beloved radio program, now over three decades old, whose sly conceits play on a homespun America that barely existed. “Real people” (Garrison Keillor, playing “G. K.,” an addle-patted caricature of his radio persona) cavort on the actual stage of St. Paul’s Fitzgerald Theater with pure Keillor inventions, like private-eye Guy Noir (Kevin Kline, badly channeling Peter Sellers) and cowboys Dusty (Woody Harrelson) and Lefty (John C. Reilly). The closing crawl thanks “The Johnson Sisters” for use of their names, attached to characters that are never other than Meryl Streep and Lily Tomlin, or at least the images they project. This is a movie so filled with self-referential winking that you could leave the Cineplex with a nervous tic.