There's far too many Ansems and Xehanorts, but that doesn't change the fact that as I see it, most of the Xehanorts we see are off shoots or so closely related that each person is completely allowed to draw their own conclusions on how best to identify them. Since, in canon, they're all Xehanort. It's splitting hairs. I've found my way, and it works. I feel that's all that needs to be said on the matter.
I'm sorry for the confusion, but officially that's how I identify those clones. It's how I trained myself.
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As for Billy Zane's career, I know nothing about the individual, nor do I care. In my opinion he was a superior young Xehanort to any of the other actors that portrayed the character. People can all have their own favorites, but none of them were very good to me. I just thought Richard was too obviously evil, and his voice had no charm. Zane's voice was smooth, almost unnervingly so for the villain.
Unless it's elder Xehanort. Then Spock all the way.
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Now, as for Terra, he was quite atrocious. His performance left me utterly disconnected with Terra. I'd rather see Matt Lanter, that's who I always imagine as Terra's actor.