I actually disagree with that quite fundamentally. I think you have to be open to the possibility that the ending could be quite different from what you originally intended. As you get to know the characters better, you may find out that they take the story in a different direction than you originally thought they would (that may in fact have happened with TT).
The danger, I think, is quite the opposite: if you have a specific ending in mind and you're hell-bent on getting there, you'll twist the characters and make them behave as props subservient to the needs of the story rather than organically to their own development.
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