As we discuss in their "Character Analysis," the four guests aren't so much complete, three-dimensional characters as they are caricatures that represent different types of people. The characterization Hawthorne employs is well-suited to this; we get simple, definitive, explicit lines that reveal everything we need to know about them. The Widow used to be gorgeous, but ruined her reputation by acting loosely. Medbourne used to be rich, but gambled and lost it all. The Colonel wasted his youth on booze and women. And Mr. Gascoigne is a ruined politician. All the clichés and stereotypes we associate with these caricatures are expected, and very much part of these characters.
No passage gives us more information about Dr. Heidegger than the third paragraph in the story – the lengthy description of the Doctor's study. By looking at his possessions, we learn an incredible amount about his character. Heidegger has a skeleton in his closet (literally!) and a host of dead patients that haunt him. That means there's something shadowy, if not downright sinister, about the man, and certainly there's more to the Doctor than meets the eye. His bust of Hippocrates raises issues of Heidegger's medical competence, philosophy, and professionalism (we talk about this more in "Symbols, Imagery, Allegory"). Most importantly, the overall mood of the study – eerie, cobwebbed, antique – conjures up adjectives that could just as well be ascribed to the Doctor himself.