When I read books in general, I usually imagine the characters in an unexplainable way: almost always, I just "put" the character there, but I can't imagine their faces or their bodies (as I do for Harry, for example).
However, some of the times, I think in the characters as being totally equal to someone or some cartoon I had seen before (for instance, I used to think that DD was like a cartoon painter I saw in a book before seeing the movie. Now, that image belongs to Lupin, I don't know why).
As for the places, like some of the characters I imagine them as places I have seen before (Some Hogwart's rooms and the Great Hall is, in my mind, one big gymnasium I know about filled with tables and chairs)
I hope it's possible for you to understand it...
![:razz:](./images/smilies/razz.gif)