Quick example of bad D.B. acting (or, I guess to be more specific, MIS-acting) comes to mind:
In one of the early Season 3 episodes, Buffy has a dream about Angel. He shouts "Go to Hell!" at her and then it cuts to a shot of him decomposing. He laughs and says "I did."
He totally blows the line. It's supposed to be Go to Hell. *I* did. As in, "Go to hell, which is where you sent ME you perky little bitch".
But he reads it as Go to Hell. I *did*, which makes it sound like he's arguing with himself.