When I read, I create a mental image of the characters that often goes beyond what the author tells me they look like. My own experiences lead me to connect them in some way to people I've known, so they become in a way "real" people. It's interesting to speculate on how I reach those conclusions, but an author needs to do more.
We can't depend on Hollywood, of course, since stars are put into roles with little regard for whether they look the way the author has described them. Paul Newman as Ari Ben Canaan, Matthew McConaughey as Dirk Pitt, and John Wayne as Genghis Khan are prime examples.
Ask a friend who's read the same book for a detailed physical description and see how much variance there is between your mental image and his. You won't need to include every detail since the reader will supply his own, but it's a good exercise to prepare for describing your own characters, broadening the scope from just what you "see" and rounding out the people who inhabit your own work.