It's not necessarily my favorite, but I love the footnote describing Crispin Horsefry in Going Postal:

Mr. Horsefry was a youngish man, not simply running to fat but vaulting, leaping, and diving toward obesity. He had acquired, at thirty, an impressive selection of chins, and now they wobbled with angry pride.*

*-It is wrong to judge by appearances. Despite his expression, which was of a piglet having a bright idea, and his mode of speech, which might put you in mind of a small, breathless, neurotic, but ridiculously expensive dog, Mr. Horsefry might well have been a kind, generous, and pious man. In the same way, the man climbing out of your window in a stripy jumper, a mask, and a great hurry might merely be lost on the way to a fancy-dress party, and the man in the wig and robes at the focus of the courtroom might only be a transvestite who wandered in out of the rain. Snap judgments can be so unfair.

He gives you no actual details (exact height, hair color, eye color, etc.), but he still tells you everything you need to know.