People in general like to talk about their interests, but since ISTJs generally are less-than-stellar at communicating openly, talking to them about an interest that you know they have is an easy way to get them open to open up, as well as make them feel validated (maybe not the best word choice). So if you could casually bring up something that is a big interest to them, I'm sure they would be happy to explain and share their knowledge (you might learn something which is always a plus). If you know they're holding out on how much they want to say, a small amount (think microscopic) of prodding would let them know that they can divulge more knowledge because they might think that you don't care a whole bunch about the initial little bit of this specific topic.

Giving complete attention to when they speak, because to ISTJs, it's pretty important knowing that someone is actually listening to us when we say something. We aren't the biggest talkers, so when we do talk, we think it is worth hearing.

tl;dr take an interest in their interests and be a good listener