Interviewing stakeholders to clearly define the problem is the most important action to contribute to the success of the initial stage of the project. Interviewing is an elicitation technique that involves asking questions to stakeholders to gather information about their needs, expectations, issues, and goals. Defining the problem clearly helps to establish the business need, the project scope, and the solution approach. References:
PMI Guide to Business Analysis, Chapter 6, Section 6.3.2.5; PMI Professional in Business Analysis (PMI- PBA) Examination Content Outline, Domain I: Needs Assessment, Task 2.