To describe a character or not to describe a character? I recently read a book that didn't have one description of their heroine. Not ONE. In the loops I follow there seems to be a growing the trend -show don't tell, but don't go into detail about what the character looks like because it slows the pace. Does this seem right to you? It doesn't to me.
We find out a lot about our characters by their appearance. Sure, you shouldn't judge a book by their cover, but if your hero is stumpy he won't attempt to jump a fence while pursuing a bad guy. He'll have to come up with a creative, ingenious solution. If our heroine has a little foxy flare she might use her charm to seduce her way into an exclusive club in order to dig up information. That sexy look may also be what gets her some unwanted attention. There are as many merits and flaws in the physical aspects of the character as there are in the mental and emotional ones. Heck, mental and emotional hang-ups could be caused by a physical characteristic.
You could create an entire story based on something physical. For instance: As a teenager Billie had terrible acne. Now she has pothole scars all over her face. No matter how many her mother reinforced how beautiful a person she is Billie still feels self-conscious. Timid as she is, Billie wants to go to her ten year high school reunion but she cannot go looking the way she does. Through a series of over the counter treatments, dermatology visits and finally an appointment with a plastic surgeon Billie gets ready to go. Then (and there has to be a bum bum bum THEN) on the day of her surgery Billie's mother dies. Of course she forgoes her surgery. While sorting through her mother's belongings Billie stumbles upon something that finally gets through to her, a picture, a poem, or a love note -whichever works. Billie overcomes her insecurities and is a smash hit at the reunion.
Don't think of a description like you did in elementary school (i.e. Bobby has brown hair. He has blue eyes. He is wearing a t-shirt with a puppy on the front and blue jeans.) find a way to work something in that flows with the pacing. Feel it. Show it. Love it. Let your characters speak for themselves in everyway possible.