The author of the novel James Dickie used several of his friends as models for the characters, and seems to have been interested in exploring the Southern US (male) personality. In making the film, John Boorman is more keen on the idea of urbanites grappling with the untamed jungle. The canoe trip is on a river that will be dammed up and destroyed. These city slickers are "raping" the natural world, in Boorman's words. And he describes the 'Mountain Men' as being like malignant forest sprites exacting their revenge. Bourgeois fears of a demonic working class aside, there is something captivating about the way the film turns a concrete situation into something resembling myth.
That isn't my description, but David Thomson's, who is otherwise rather down on the film's simplistic message and stereotyped characters. I think that although Burt Reynolds is sometimes a little melodramatic, the acting in general is superb, and fills out the limited characterisation. Moreover the details in the film – Bobbie's ribald jokes being turned against him, the church moving ground as moral convictions are shaken, the ambiguity of how people die – is impressive. It's a lean, expertly put together thriller, even if the metaphor driving it is hammered home within the first five minutes.