Sandra hated Bob, since the first day he started working in the lab. She looked him up and down. He had reddish hair, green eyes, and a bad complexion. He wore jeans and a blue polo shirt. She never liked how he looked at her. He always seemed to be a know-it-all, and he always had a smirk on his face. They began to fight.
So, what do you think of the opening paragraph? I agree, it's awful. It's a classic case of telling instead of showing.
One of the methods I use to see if a story needs to be tweaked from telling to showing is to check for the laundry list approach. Looking at the opening paragraph, we can see a list of Bob's physical characteristics, followed by a list of his clothing, and finally a list of why Sandra hates Bob.
Sometimes it's fine to put up a two or three item list, if the story calls for it. For example:
When she met Bob, she thought, "Short, redhead, stupid and cheap."
It's a good, quick first impression, not a huge catalog of personal traits.
So, how can one divulge this laundry list of information without just listing it? It's easy, if you embed the information in the dialogue and action, like this:
"You should lower the flame on the Bunsen burner, Sandra," Bob said, his face twisted in his usual vapid smirk.
Sandra had enough. Ever since the weasel-faced know-it-all had started working in the lab, he second-guessed every decision Sandra made. "Bob, get out of my face before I punch some of your pimples out."
His green eyes darkened, his face reddened to match his hair. "I don't have to take that from a hack who got her degree by sleeping her way through organic chemistry!"
Sandra dove over the sink, knocking a small vial of carbon tetrachloride onto Bob's Wal-Mart polo shirt, staining the blue to a darkened grey. Bob tried to leap back, but his short legs weren't strong enough to get out of harm's way. Sandra's well-toned arm came down quickly, knocking her tormentor unconscious.
You see? The information is all in there, but it's spread out so the reader isn't subjected to a list. Of course, this could be spread out more. You can drop descriptive elements in different parts of the story. For example, his complexion is needed here, but the description of his eye color could be moved to another section or chapter -- say when they fall in love. Give the information to the reader when it's needed, not when it's convenient for you to write. Build the character pebble by pebble, not with an avalanche. It makes the character believable if the parts get added slowly, since it slightly modifies the reader's mental picture until the image is complete and accurate.