Writers of writing self-help books, please stop giving out the advice to eavesdrop on those around us when we’re out in public in order to form a good model for writing dialogue. Eavesdropping doesn’t work.
I have tried eavesdropping multiple times when I’ve been out at restaurants or in stores. Not only is there almost always music playing, and very loud music at that, but the rumble of ambient noise fills in all the quiet spaces between the words my target subjects are saying. In restaurants, add the ting, crash and clink of glassware, dishes, and tableware. The only place I’ve found that I can reliably eavesdrop on people is listening to the characters in television programs and movies, and that’s already dialogue.
In future, admit that eavesdropping is impossible. It will save those of us trying to develop dialogue a lot of trouble, and we won’t think the rest of your advice is as valid as the recommendation to eavesdrop.