Requirements gathering is 'THE CRITICAL' foundational activity for building the data model(s) for a business intelligence environment. I do agree not all requirements can be captured and documented when we start but still a detailed requirements gathering approach is likely to yield information to help in developing the initial design. Off-course it will help to extend if additional needs are identified. It also help to map the "real requirements" and explore them instead of getting deviated with what is provided. 'Real World' is driven by perception and we have to live in this real world.
When we want to capture the requirements we should prepare ourselves for the same. It includes understanding the Scope, business issues, collect relevant documents and 'Who & How' going to provide information.
Project Scope - a Document describe what’s need to be delivered and what can be ignored. It also mention minimal project timeline and the tentative available resources. Off-course it contains major issues and assumptions to document risks.
Business Issues - Data analyst should have in-depth understanding of information gathered and its mapping to corporate strategies. Analyst should understand what are problems which need to be addressed.
Who and How - Any information need to be solicited from variety of people in roles including the sponsors, steering committee members, business SME(s), business analysts and end users. Apart from these people involved in providing decision support information or people familiar with possible data sources. Post identification of these people discuss with each of the individuals or groups to collect the information in a manner they are more comfortable. Information gathering is a technique and it requires to be tailored to each participant. Need to be well planned sessions with crisp queries to avoid missing the opportunity in time allotted. But need to be flexible enough to handle situation and get answer in different format. Discussions should be documented and if any follow-up commitments should be completed.