Conducting a requirements analysis interview

Question

Why do requirements interviews never pan out as they are planned - and how do I prepare for that? I'm looking for the following kind of help regarding how to prepare for these interviews and what I can pay attention to in order to make the most of them.

My information project, centered around management reporting within a firm, is currently in it's definition stage. All my stakeholders are managers of greater or lesser scope.

I had my previous two interviews planned out quite extensively, but on both occasions my interviewees both went off in completely different tangents, delivering quite interesting results. I completely ignored the question flow I had prepared, and tried to go along with the interviewees. But instead of going towards a common concept that can be developed on, I feel that I get in-depth pictures of each individual's local information requirements.

How can I conduct an interview that helps me get the participants on board and thinking towards a common solution, but doesn't inhibit their own thoughts which deliver me interesting material on data sources, formatting and communication paths?