Drew and the Banjo boy clearly develop and enjoy a relationship while they are playing.
The music is almost like dialog, like call and answer, they react to one another and it represents a rare moment of optimism in what is an otherwise unbearably dark, oppressive film.
All of this is despite an atmosphere of intense mistrust and power imbalances. The scene starts as one of conflict. The city professionals have got lost on their way to the river where they are hoping to enjoy a canoeing expedition. They come across a local community who are referred to as “rednecks” and the locals suspect the visitors of being businessmen intent on damning the river, flooding the valley and evicting them from their homes.
Drew and the inbred Boy playing the banjo create a platform of mutual understanding, both players start to give a little more. There is a meeting of minds and cultures, which is suddenly optimistic and shown to be almost contagious.
But the neglected handshake is a foreshadow of what follows or what the movie is about. A handshake would have established a relationship or recognized them as equal, but they are not and about to find out as there is no deliverance in Deliverance.