Bernie is Bernie Tiede, an assistant funeral director in Carthage, Texas, and the movie's his story. It's all pretty much true. I read the original article about Bernie in Texas Monthly, and the movie follows it right along. Bernie befriends a wealthy widow, played by Shirley MacLaine, and things go downhill. If you haven't read about it, then that's all you need to know. It's like every noir story you've ever read, except it's not.
A lot of roles in the movie are filled by residents of Carthage, and they're all wonderful at just playing themselves in documentary-style interviews. They'd steal the show if it weren't for Jack Black, who's note perfect all the way through. (Another reason I liked this movie is that I know someone who's just exactly like Bernie. Exactly.) MacLaine is good, too, and so is Matthew McConaughey in a small role.
I recommend this highly, especially if you're from Texas.