Pioneers and high-hour publishers who tend to work in midweek field service form a gossip network. If you want to know what is going on in the congregation, go out in midweek field service with a gossipy pioneer and the whole car group will soon know, and then everyone else.
This is an unintended consequence of this culture. When a person does tedious task with little to show for all the work, going from empty house to empty house, riding from territory to territory to random return visits, and finding creative ways to make one’s time, eventually people run out of things to talk about. Gossip is an inevitable result. Anything that could be juicy or mildly interesting. It is just human nature, no matter how counseled against.
If the public knew how many records JWs took on them, they would be alarmed. Even if a person doesn’t answer the door, their address can be taken down on a not-at-home slip. If a person takes a magazine, a JW will write any detail about that person so they have details in their return visit book for later conversation. It's almost a "little black book" of interested ones.
Any perceived interest from a non-JW can be the subject of an “experience” that can be related to other JWs in the car group or at the Kingdom Hall when experiences are called for. Any detail a person brought up might be mentioned. Sometimes a JW will brainstorm with other JWs on how better to reason or reach such a person. No topic is off the table, no matter how private.
Of course, a “Bible study” will be talked about as well. As the expression “it takes a village to raise a kid”, the congregation mobilizes to help sort of raise a “spiritually immature one”. You are viewed as a spiritual child and they will try to “guide” you to “maturity”, that is to think and act just like they. As parents discuss a child, so too they can discuss too. Anything you’ve told your study conductor is easily fair game for discussion with other JWs.
In a cult, there is no such thing as privacy.