Personally, I like to find what people are interested in, and let them talk about that. I tend to learn things that I didn't know before, which is always interesting. For example, yesterday I was sitting by a guy who started talking about watching the British Open. I don't care about golf. At all. Never played, never even watched it on TV. But through chatting with him, I learned that it's technically just 'The Open' (and then we Americans started calling it 'British' to differentiate it from our newer version), that bunkers there tend to be much deeper than in the US (since they're by the ocean, and the strong winds would blow sand out of shallow ones), and a few other things. I found it interesting, even if it's not knowledge I'm likely to use, because he was interested in it.
If you try and peel back a layer of what you think is most important, but they don't care about it at all, you won't find anything worth hearing. There's nothing there, because they've probably never bothered to think about it. So find something that they have bothered to think about, and learn what you can about that.
You do have to avoid judging others, though. If you decide someone's interest has no worth, then it's going to be a boring conversation. It takes either empathy, so you can see the world through other's eyes and understand why they're interested in it, or just having a view that any learning is worthwhile.
If you try and peel back a layer of what you think is most important, but they don't care about it at all, you won't find anything worth hearing. There's nothing there, because they've probably never bothered to think about it. So find something that they have bothered to think about, and learn what you can about that.
You do have to avoid judging others, though. If you decide someone's interest has no worth, then it's going to be a boring conversation. It takes either empathy, so you can see the world through other's eyes and understand why they're interested in it, or just having a view that any learning is worthwhile.