No one much likes Marjorie; even her family haven't spoken to her in years (and that includes the sister who lives in the same town). But she takes a liking to Bernie, and the two begin spending a great deal of time together -- going to concerts, traveling around the world on expensive vacations (Mr. Nugent had made a fortune in oil).
Ultimately, though, Marjorie's inherent nastiness wins out, and she gets more and more demanding, expecting Bernie to be at her beck and call. He reaches his breaking point and kills her, then manages to keep anyone in town from finding out she's dead for nine months. That's when Matthew McConaughey enters the story as the district attorney saddled with the task of convicting the town's most popular resident of killing its most hated.
In addition to the fictional telling of the story, Linklater has gone to Carthage to interview the locals about the case, and nearly half of the movie is made up of their running commentary on the events. These scenes aren't specifically identified as documentary; it feels more like a particularly colorful group of unknown actors playing a sort of Greek chorus.
That type of footage can, in the wrong hands, can come off as condescending -- "oh, aren't these yokels colorful" -- but Linklater almost always avoids that, in part because the people he's put on screen are so likable and charming, and their interviews are skillfully edited into the movie. The locals often provide a sharp punchline at just the moment it's needed to break the tension, and some of them have such natural charisma that you'd almost like to see them in movies of their own.
Jack Black is terrific in the leading role, in a gentler and more charming performance than we're used to from him. And it really is his movie to carry; MacLaine and McConaughey are both fine, but their roles are relatively small.
Bernie is a strange movie, and I occasionally found myself wishing it were slightly less subdued and more energetic. But Black is delightful, the locals are a well-used storytelling device, and I was entertained throughout.