I am just starting working seriously on my writing, and I do go into coffee shops, eavesdrop on others conversations, and go home and write down the different ways people start and navigate a conversation.
This has helped me begin to understand how real dialogue worked, but it wasn’t enough. Before I could still write a conversation, and I had to ask WHY. Why does this character say this thing? Why did that character reply like that? How did they arrive on this subject in the first place?
I eavesdropped on conversations for a few months. It did seem a little creepy at first, but I wasn’t leaning over trying to hear. All I was doing was hearing voices that people were vocalising on the next table. It was difficult not to hear them. But it’s taught me so much about how real dialogue works.
For instance, real people do say random things.
As writers, we want our characters to talk about things central to our plot, but humans are pretty weird. They don’t talk about important things. More often than not, they talk about mundane things like the weather and the fact that their football team lost a match last week.
To write realistically random dialogue without losing track of your plot, have your characters begin a conversation about something random, and then circle around to the critical parts of your plot. But, don’t just have a full conversation on the weather today.