I tried generating some new lines for Book of Boba Fett and it took ages to get anything remotely passable. It’ll get there.
I do find, though, that putting the lines you want in quotation marks and adding something like “I said slowly and menacingly” after that helps (if you use “he said” the AI tries makes the voice you want then narrator and they deliver the actually line in a slightly different tone so it doesn’t quite work)