Have you noticed that when Dumbledore's eyes are described in fanfic, it is always in terms of whether they are twinkling or not twinkling? This expression is used in the books several times, true, but I think using it every time Dumbledore appears in a story is overdoing it a little, especially if it goes to the point where Dumbledore never expresses emotion in any way other than by twinkling or not twinkling his eyes.
I hope people realise it's a metaphorical expression, since eyes, unlike stars, can't really twinkle. When the Headmaster's eyes are described this way in the books, it is Harry thinking they looked almost as though they were twinkling, but they weren't literally, I hope! And it isn't possible to twinkle your eyes at will either – if you don't believe me, do sit in front of a mirror and try.
Twinkling eyes is a generic description. Its meaning is actually vague and ambiguous: what emotion is behind the twinkling? Joy? Delight? Devious scheming? Or are the eyes twinkling with tears?
I'm asking authors to use some variety in their descriptions of characters' facial expressions, please, because I think I've developed an allergy to repetition. And I don't mean replacing twinkling with sparkling or glittering (gleaming apparently has an evil connotation in Harry Potter), though that would be a good start.
As a beta reader, I'm sick of rewording things like his eyes twinkled like mad. Next time you're tempted to use the expression twinkling eyes as though it summarises everything you want to express, why not conquer your laziness and write instead, There was a scheming look in Dumbledore's eyes or Behind half-moon spectacles, his eyes showed a deep amusement? And what about replacing Dumbledore's eyes weren't twinkling anymore with There was a grave expression on Dumbledore's face?