After Humphrey imbibes the tonic, his voice changes to a much deeper register. The voice is provided by Thurl Ravenscroft, who was also the voice of "Tony the Tiger" for Kellogg's Frosted Flakes. Kellogg's was a sponsor of "The Beverly Hillbillies."
When Elly May tosses Jethro into the front foyer from the living room, Jethro is seen sliding across the floor. If you look closely you see Jethro sliding with a rolling device under hin.