One of the hardest things for me as a writer is to describe the physical appearance of my characters. I grudgingly understand the need to do so, if only because I’ve had readers complain that they don’t know what my characters look like.
In my mind, stories are all about what the characters do and why they do those things, their inner thought process.
Physical appearance is secondary, but it does matter when it influences story. To use a crude example, if you lure a good husband into cheating on his wife, it would help to understand what it was about the other woman that tempted him. This should only go as far as needed to tell the story. Was it her youthful appearance, her large green eyes, or the subtle curve of her bust line?
Give the reader enough to A) fill in the blanks with their imagination and B) understand the characters’ actions. If you go into minute, clinical detail on everything about this hypothetical woman that made her sexually alluring, then you’re just being a perv.
For a negative example, I have to cite the first Kinsey Millhone novel by Sue Grafton, A is for Alibi. It’s her only book that I have read, so I can’t say whether she has improved in this regard. Whenever she introduces a new character, Grafton seems to stop the action to lavish us with a physical description that person that seems to reach the pores on their left buttock, as if she were describing the person to a police sketch artist. It’s unnecessary but, because it was her first novel, I hope it was a tendency that she grew out of. I’m surprised that professional editor let it pass. Not to suggest that it had something to with Grafton being the daughter of another successful crime novelist, but it’s been known to happen.
The level of description in Grafton’s book is not the problem, but way she delivers it, hitting her reader with an information dump that brings the pace of the story to a crawl. You are better off ladling the details into the story with the action. Tell your reader about the bulk of a man’s body as it eclipses the light in the room. Describe how a girl fluttered her big green eyes at a love-struck teenager, or tell how his slight frame quivered when she touched his face.
Obviously if your character is physically distinctive in some way, you should highlight this, but only to the degree that it is relevant to your story. In an early, unpublished work, my hero was a mountain of a man with a mop of fiery red hair and a weather-beaten face, and that’s all you know about the outside.
Humans are visual creatures and when people read your stories, they are picturing the action in their head, and you want to furnish them with enough information that they are picturing it your way. However, reading is ultimately a collaboration between the writer’s words and the reader’s imagination. Over-describing is a failure to trust the reader’s imagination.