Jones plays an uninhibited version of himself. As I had occasion to discover a number of years ago during the brief run of his starring turn in Gabriel's Fire, he has a decent comic instinct. The BBT episode, though, went far beyond any funny business he did in that earlier show. He turned into the world's biggest kid in the course of the story, and although I generally dislike the adult-as-big-kid trope, his glee was too delightfully infectious to resist.
The episode is evidence that the show hasn't entirely lost its way. When Bernadette, and even more so Amy, came on board, the show changed direction to focus on their relationships with their respective boyfriends. The characters are all quirky and unique but relationship humor isn't, and my interest quickly waned when that became the center of the writing. The James Earl Jones episode finds the characters' relationships stable enough that they can talk about other things instead. It's a relief. May their geek flags fly freely once more.