I think the writing and acting have been perfect for making Willie seem like a dangerous coiled spring, which is exactly what they were going for. That's exactly how Willie started. Later, of course, we get to hear both actors' reading of the same crossover scene, and Karlen seems too subdued.
[spoiler]It's almost as if someone knew they had the perfect actors for pre and post-crypt, but no actor perfect for both, and decided to use both actors. I doubt it, but still... The perfect crossover between Willies, then, would have been when Barnabas reaches up to strangle him... then reaching up another foot or so, and reshaping his face entirely, while he's in the neighborhood![/spoiler]