What is the most important thing you've learned about interviewing?

1. Tell the interviewee what they want to hear up front Who are you? What type of interview is this? How long will this last? Will I be able to ask questions? What's the next step after this?
2. Don’t ask leading questions. For example: “How do you work with engineering” gives away that you’re interested in their collaboration style with engineering. They will always answer this question with something like “I am really communicative with them and do all the right things!” To get the real answer here you want to ask an open-ended question like “tell me about x product you launched” and in their response see how they naturally bring up engineering. Or, they may not bring them up at all and you may have your answer or something to dig into more.
3. Dig deep in 1-2 areas. With a product case I like to dig deep in 1-2 ideas on their roadmap or challenge the metric they’ve come up with. The purpose of doing this is to see how the candidate reacts to being challenged and to see how deep their thinking was behind something. I’m looking for density of thought when I dig. In a behavioral interview, I dig deep on 1 past experience or feature they built. Why did they build x feature? How did they come up with the idea to build it? What was the launch strategy? How did it do? What was the design process?

Talk even less and give the candidate even more time/space than you think to thoroughly respond to your question. You'll learn a ton about a candidate simply by giving them the floor.

Also - it's a lot harder than it seems.

(There's a 300 character minimum to these responses, so I have to keep typing to hit the minimum, even though I already answered the question. Hopefully this gets me over the minimum!!!!!!!!!!!)

The best way to assess candidates is by being a good listener. The lessons learnt from successes and failures are something that one should listen to.