I once belonged to a reading group where one member, no matter what book we were discussing, would invariably ask, “Who would you cast as . . . ?” In all fairness, he was a screenwriter, but his perennial need to graft the face of some Hollywood star onto a given character in a novel could be irritating. As I read Peter Mendelsund’s quirky and fascinating What We See When We Read, I came to the realization that this casting device may have been this reader’s imperfect way of visualizing what he was reading.