When I write a character, e.g. Draco Malfoy, I don't just extrapolate their mind, I extrapolate the surrounding subjective world they live in, which has that character at the center; all other things seem important, or are considered at all, in relation to how important they are to that character. Most other books are never told from more than one character's viewpoint, but if they are, it's strange how often the other characters seem to be living inside the protagonist's universe and to think mostly about things that are important to the main protagonist. In HPMOR, when you enter Draco Malfoy's viewpoint, you are plunged into Draco Malfoy's subjective universe, in which Death Eaters have reasons for everything they do and Dumbledore is an exogenous reasonless evil.

This is an awesome trick, and I'll have to use it more explicitly when writing various characters. (I already did somewhat, but I'm not sure if I've explicitly thought of it in these terms.)