Director Robert Altman and screenwriter Garrison Keillor have turned the beloved old-timey radio revue A Prairie Home Companion into an enormously entertaining backstage comedy that's wall-to-wall with country-and-western music. The movie takes place during the fictional last performance of the show, which has been canceled by a Texas media conglomerate, and as the numbers play out onstage, an angel of death in a white trench coat (Virginia Madsen) wanders the wings searching for her prey. These plot elements may be hokey, but they provide an adequate structure for some of the funniest and warmest character improvisations Altman has generated in years. With Kevin Kline, John C. Reilly, Woody Harrelson, Meryl Streep, Lily Tomlin, Lindsay Lohan, Maya Rudolph, Tommy Lee Jones, and Keillor, who turns out to be an imposing screen presence in his own right. PG-13, 105 min. Reviewed this week in Section 1. Century 12 and CineArts 6, Esquire, Lake, Landmark's Century Centre, River East 21.