What umptions do you make about characters in books?

Not so long ago I lent a book called 'Anansi Boys' to my cousin and she didn't realise until she was halfway through that all the characters where Black British unless specified otherwise. This got me thinking. When you read a book, do you immediately ume the character is the same race or ethnicity as you? Do you ume they are white? If you are British, do you ume they are all British and the book is set in the UK until told otherwise? (likewise for people of other nationalities - do you ume they are from your country?) Or alternatively do you see only their personalities and not really their physical description or regional -isms?