Sometime in the early 1980s, I saw Richard Burton in a touring production of Camelot. He sleepwalked through the entire thing until the speech at the end where Camelot is in flames. For one brief, shining moment, he was magnificent.
I think Liz f*cked all the creativity out of him, at some point he became a parody of himself and just went through the motions of being an actor. Self-disappointment at the never-reached potential.