The "inbred banjo boy"

YouTube link.

The actor is Billy Redden from Rabun County, Georgia who was 15-years-old at the time. Redden was "discovered" during a casting call at his school in Clayton, Georgia. Special effect makeup was used to augment Redden's already-unusual features. He didn't actually play the banjo. According to The New Yorker, "Boorman had had to deploy another boy to hide behind the swing and slip his hand through Redden’s sleeve to finger the changes."

Further details, and a video interview with the (normal-looking) adult Billy Redden at Boing Boing.