The whole cast, down to banjo boy and the people in the opening scenes, to the doctor and nurses at the end, were extremely well cast.
I grew up with and knew hillbillies in the hills west of Austin. There is even a book written about them. We told them about Deliverence and they scoffed, as suspicious and critical about it as everything else in the outside world, that it was probably something like Green Acres or Hee Haw. And then they went and saw it.
They rooted for the hillbillies and picked out subtle things one would notice when watching your side perform well, like when the hillbillies were leading Burt Reynolds to their destination and one hillbilly jumped in riding shotgun, literally, and stuck a dbl barrel shotgun in the window rack of the winch truck they were in because they knew not to trust the city folk.
Then the rape scene. They all hooted and laughed with great pride afterward how the hillbilly had to back out a foot when he got finished railing Bobby in the ass.