I've been doing a lot of critiquing lately and have come eyeball-to-page with this ugly symptom of first drafts. If your MS is a face - a perfect, symmetrical face of beauty - then overwriting is like an overbite. As soon as your story opens its mouth, overwriting is revealed. Readers are caught staring at it, rather than the whole story.
Oy. That was disturbingly easy to write. Those sentences are littered with adjectives/adverbs that are another form of telling. We get carried away by our own awesomeness, by the idea we can paint a picture with words instead of what isn't said. Let me say that again: what ISN'T said. The most effective writing is one that gives the reader enough info to populate his/her own imagination. It's a fine line and one that can years to perfect. Let's see if I can demo this right off the cuff:
"He grimaced so she saw his pointed teeth. Tears snaked down her cheeks, hot trails of fear fueled by the knife at her neck."
Far fewer words, yet a much bigger impace on the reader. Or so I hope. You'll have to tell ME which is better. (This is a subjective business, no?)
Too many stage directions. "Hey," he said, sitting down next to me on the top stair. He put his backpack down and crossed his left leg, angling himself so he faced me.
"Hey," I said back, giving him a half smile so my dead tooth didn't show. I combed my hair with my fingers and then braided it into a single braid as thick as my wrist. "What's going on?"
"Not much." He rummaged in his backpack for an apple. It was red and had a bruise. He held it out to me with his right hand. The fingernail was broken on his index finger.
*sounds of choking* Um... I don't care how they're sitting or if someone has a mole on their left butt cheek. (Unless the mole is somehow diabolically related to the plot in an intrinsic way.) I want to read the dialogue or experience another plot point, not feel like I'm trapped by every single move the MC makes. Good writing is all about the right kind of detail. Constant updates on the way somebody combs their hair usually don't fall in that category.
Those are my three...and I wrestle with them often. What's your idea of overwriting? Please share with examples in the comments.