Director Robert Altman thinks it’s about death. Writer Garrison Keillor thinks it’s a light comedy. They’re both right. But this screen take on Keillor’s three-decades-running public-radio show, A Prairie Home Companion, is something else as well: a delectable antidote to the hard-sell sideshow of Hollywood. Prairie goes down so easy that you probably won’t notice at first how artfully it’s done. Take your time and relax into a movie that’s as comfy as Keillor’s pillowy baritone. Minnesota’s Keillor has concocted a death-wish fantasy, imagining that it’s curtains for his radio variety show, broadcast from the Fitzgerald Theater in downtown St. Paul. A big corporation from Dubya’s Texas has sent in an ax man (Tommy Lee Jones). What we’re seeing is the last performance with Keillor, as GK, playing ringmaster onstage and off. Camera whiz Ed Lachman bathes this valedictory in shimmering color and light. Altman, as he has from Nashville to Gosford Park, assembles a dream cast. A luminous Meryl Streep, playing it loose and funny and true as singer Yolanda Johnson, fits in snugly with an ensemble that includes the treasurable Lily Tomlin as her sister Rhonda. Their duet on ”Goodbye to My Mama” is a beaut. Altman says his movies are all in the casting; he just watches. Don’t buy it. Other filmmakers who try to match his alchemy with actors fall flat on their fat ones. Altman, 81, is still a master at the top of his game. He even brings out the best in Lindsay Lohan as Yolanda’s daughter Lola, who writes suicide poetry. Lohan rises to the occasion, delivering a rock-the-house version of ”Frankie and Johnny.” The songs, sung live with Keillor’s Shoe Band, exude an uncanned charm. They can also be howlingly comic when warbled by John C. Reilly and Woody Harrelson (”I’ll give you my moonshine/If you show me your jugs”). When the sexy angel of death (Virginia Madsen) slips past security, in the person of Kevin Kline’s Guy Noir, the plot thickens. Not too much, though. Keillor loves corn but not syrup. After a performer dies backstage, Yolanda asks GK for a moment of silence. His response: ”Silence on the radio — I don’t know how that works.” I don’t know how this movie works either, only that it does. For those, me included, who used to think of Keillor’s radio program as tepid, self-indulgent, repetitive and flat, you might even call it a revelation. Take a swig of this moonshine. There’s magic in it.