Writers, like everyone else, can—and often do—use wait-time to read, check Facebook, etc. But writers have so many more options!
Things to do during any wait
Practice describing: Choose any one person and describe him/her in detail, and as vividly as possible. For the most benefit of this practice, try describing both stand-out characters and those who look as ordinary as possible.
Practice judging a book by its cover: Which is to say, consider another waiting person and, on the basis of what s/he is wearing, imagine socio-economic class, education, type of job, personality, and anything else that comes to mind.
Practice noticing non-verbals and extrapolating from them: If a man is fidgeting, repeatedly checking the time, etc., maybe his marriage is precarious, and he’s imagining confronting his wife, saying, “I swear! I was standing in the post office line the entire time!”
How are the waiters behaving? Is there generally patience, politeness, and/or acceptance? Grumbling, swearing, people leaving? What might contribute to the ambiance? Size of community? Geography? (E.g., Richmond vs. New York City.) Consider what would happen if someone behaved differently from the majority.
Practice disrupting the status quo: Do something unexpected, even if minor, and observe the responses around you. For example, sway from side to side, pat the top of your head, march in place, etc., and keep doing it. Don’t make eye contact—and don’t laugh!
Practice introspection: When stuck with a long wait—longer than expected—how are you feeling? Check your visceral reactions: breathing, muscle tension, heart rate, stomach. Are you relaxed, tense, bored, impatient, or something else? What are your inclinations—stay, leave, sigh audibly, complain loudly, try to jump the line, seek redress with someone who seems to be a gatekeeper, or something else? Why do you act on those inclinations—or not?
Things to do while waiting when noise isn’t an issue
Listen to ring tones: Try to identify them, or at least get the rhythm, and extrapolate from that their personality.
Practice eavesdropping—and spin a story from it: A woman says, “I noticed that your wife is wearing orthopedic boots.” Man says, “She has diabetes, and doesn’t have any toes on her left foot. She doesn’t have a big toe on her right foot. The boots are so she can try to balance.” Listen to mobile phone conversations and proceed as above.
Incite responses: This is in line with disrupting the status quo. Say something outrageous! You can say it to someone else in line or you can pretend to have a mobile call and let others in the line “overhear” you saying something outrageous. For example, “I’ve had sex with thirteen men—and, no, I’m not promiscuous!”
Opportunities in specific places
Airports: Where is s/he going? Why? What’s in his/her carry-on? Traveling coach or first class? Why was s/he pulled aside for further security screening? Is this person traveling alone or not? Is it a family? Business colleagues? Lovers?
Doctor’s/dentist’s office: What’s his/her condition? Is it terminal? Does that bald person have cancer? Does that person have reason to be nervous or is it just “white coat syndrome”? If the former, what reasons? If the latter, what is the origin?
Grocery store: Check out the carts around you. Is this person shopping for one or a family? Is this a health-food nut or a snack food junkie? Omnivore or vegan? What does it say if the other shopper brought bags, asks for paper, or goes plastic?
BOTTOM LINE: Use your waits to build your writing arsenal!