From A Writer's Workbook by Caroline Sharp

Stop 'n' Shop:

All you need for this exercist is a pencil, paper and your nearest supermarket. Make a list of shopping cart ingredients. Start with a relatively small group of items. See if you can come up with ten different cominations without repeating. Give yourself ten character profiles and write up their shopping list. Pick out a single item and fill in around it for as many different customers as you can.
Don't be afraid to make huge assumptions about these people based on what you see them buying. You can judge these shoppers just by what is in front of you right now. Why does the old guy ask for regular milk on Monday, 2 percent on Wednesday, skim milk on Friday and heavy cream on Sunday? Stranger than fiction.