Some of the actors are virtually spot on my imagined pictures of them, and as they are drawn largely from the ranks of the great and good of the British acting profession, I can imagine them 'becoming' the characters. I had not imagined Harry's Uncle being so old, but I have the highest regard for Richard Griffiths, and can imagine him becoming Uncle Vernon very easily, likewise Alan Rickman as Snape, and Maggie Smith - practically anybody she wants to be.
Nobody should make the mistake of expecting the film to be as they imagined the book. It is merely an interpretation. Many actors have portrayed Fagin in "Oliver", and "Oliver Twist": Alec Guinness, Ron Moody, more recently Robert Lindsey. None of them really came close to my image of Fagin from the book, (who was a really dark evil character) but the interpretations were all excellent nontheless. David Lean's "Great Expectations" is generally considered to be one of the great British movies, but how close was it to the novel? Just the bare bones, interpreted.
I can't wait to see Harry Potter at the movies, and I don't care if it isn't how I imagined it.