My favorite piece of writing advice is this:
When my creative writing professor first recommended it to our class, I felt as though a huge weight had been lifted from my chest. I had been in the habit of listening in on other people for years, and always felt pretty guilty about it. As he spoke, I suddenly realized that there was nothing to feel guilty about. It wasn’t eavesdropping, after all. It was research!
So, why spy?
Listening in on other people’s conversations can give you inspirations for new stories and new characters, for one thing. More importantly, it gives you a better understanding of natural speech cadences and dialog progression. There is nothing like listening to two people having a real conversation to make you realize three key elements of writing interesting dialog: