While Richard Griffiths was ~basically~ the perfect Uncle Vernon, Harry Melling and Fiona Shaw both lacked the blonde hair that JKR mentions pretty often in the books.
“Mr Dursley … was a big, beefy man with hardly any neck, although he did have a very large moustache.”
– Harry Potter and the Philosopher’s Stone
“Mrs Dursley was thin and blonde and had twice the usual amount of neck, which came in very useful as she spent most of her time craning over garden fences, spying on the neighbours.”
– Harry Potter and the Philosopher’s Stone
“Dudley looked a lot like Uncle Vernon. He had a large pink face, not much neck, small, watery blue eyes, and thick blonde hair that lay smoothy on his thick, fat head.”
– Harry Potter and the Philosopher’s Stone
Evanna Lynch was perfect for Luna personality-wise, but her platinum-blonde hair didn’t quite fit the description of Luna’s “dirty blonde”.
“She had straggly, waistlength, dirty blonde hair, very pale eyebrows, and protuberant eyes that gave her a permanently surprised look.”
– Harry Potter and the Order of the Phoenix
Ralph Fiennes was already pretty terrifying as Lord Voldemort, but he would have been even scarier if he had had book-Voldemort’s “livid scarlet eyes”.
“The thin man stepped out of the cauldron … Whiter than a skull, with wide, livid scarlet eyesand a nose that was flat as a snake’s with slits for nostrils…”
– Harry Potter and the Goblet of Fire
And finally, there’s no arguing against it: Richard Harris literally was Dumbledore.
“He was tall, thin, and very old, judging by the silver of his hair and beard, which were both long enough to tuck into his belt … His blue eyes were light, bright, and sparkling behind half-moon spectacles and his nose was very long and crooked, as though it had been broken at least twice.”
– Harry Potter and the Philosopher’s Stone