One of your greatest weapons as a writer is your ability to observe. People, places, objects, atmosphere, snippets of dialogue – they’re all material. But observing well in public is no easy task.
Imagine: you’re at a diner, and you notice a couple a few tables over. They don’t speak, yet their silence is eerily comfortable, as if they’re conspiring together. You're taking in their subtext when... BAM, the woman shoots you a scathing look. Why? You stare!
So, how do you go from open-mouthed gaping to cloak-an-dagger espionage? You must learn the art of indirect observation: (1) Position yourself close. Hearing is often more important than seeing – accent, cadence, tone, dramatic pauses in conversation. (2) Use props: a book, a cigarette, a pen - any object where you can focus on the part, yet look beyond the foreground. (3) Express an emotion – pensive, angry, frustrated – as you look past your subject. (4) Use mirrors. Lots of public places have mirrors, which allow you to watch what’s unfolding behind you.
Now, this is not to say I'm advocating wiretaps, bugs, or back room monitors. To become known as the creepy "stalker" guy at the corner table is no achievement you should be proud of. It takes skill and practice to master the subtle art to constructive eavesdropping. So listen and pay attention to your environment: the guy at the DMV, the couple at the airport, the teen lovers at the movies. You never know what snippets of dialogue or action that might become the next golden nugget in a character, a story, or a screenplay.