JWs used to (still do??) get a basic list of talking points for overcoming objections or answering simple questions about the Bible. The Reasoning book compiled these by subject, giving the rank-and-file a way to respond to many different questions or objections, which gives them the confidence to 'defend' what they believe. They are also taught not to engage if the discussion becomes contentious or if they perceive that they are being preached to (ironic as that may sound).
Keep in mind that most people know about as much about their religion and texts as the average JW. I found most Christians that I encountered knew surprisingly little, so that even the surface-level preparation that I'd gotten from the WTS made me feel as if I had a wealth of knowledge compared to them. Many people are content to serve and worship without getting a more in-depth understanding of their religion. By that standard, many JWs are quite well prepared to have a short debate on almost any Bible subject.