Quick and Dirty: No time for prep? No time for rules? No problem.
From the Dice of Life blog:
"Anthony came over to play with us one afternoon. He'd played D&D as a kid, and his parents still played regularly, but he hadn't rolled the dice in years. I was worn out from work, and it was kind of impromptu thing, so I didn't have anything prepared.
"No one wanted to spend an hour or more rolling up characters for a one-shot, so I did what I do best: I improvised."