(Note: After several years of successfully dodging colds, I finally got caught by one. I'll live, but the accompnaying sore throat is making it tough to use the VRS for any length of time. Posting will likely be delayed or late until I get my voice back.)
One-liners as characterizations is a traditional form of verbal short-hand in the southern U.S. We considered it witty to offer a short anecdote or observation on a person in such a way that can be later expanded into a proper yarn, if need be. Most folks dismiss them as sayings or colloquialisms, but while they're usually joke-funny, they're also often painfully accurate:
She fell outta the ugly tree and hit every branch on the way down.
He's busier than a one-legged man at an ass-kicking contest.
I don't know what sent her over the edge, but in that family, it doesn't take a real hard push.
He's got two ways of fixing things: do-nothin' or duct tape.
That girl would screw a snake if you held the head.
Most often the one-liner characterization is best delivered in dialogue, as it is an observation or gossip, but it works in the narrative, too. It takes a little investment of trust in the reader to "get" a description that isn't a typical recital of physical attributes, but with the right words you can prompt the reader's imagination to fill in the blanks.
With this sort of characterization, there is the temptation to use an easy, cliche analogy: as fast as greased lightning, dumber than a post, crazy as a fox. You can use cliches as practice by taking them, shaking them, and turning them into something new, like as fast as TV preachers go to Hell, dumber than invading Antarctica, and crazy as a dog after a treed squirrel.
You don't need to resort to the classic analogy form for a one-line characterization, either. Claes, a character who could rightly be described as a tall, sturdy, muscular, brown-haired youth who seemed immovable and unbending, yet who still possessed adorable, boyish indentations in his cheeks becomes in Dorothy Dunnet's hands an oak tree with dimples (Niccolo Rising.)
Your assignment today: in comments, give us a one-line characterization describing one of your characters, or a character from your favorite book.