As a business Analyst, your primary responsibility is to collect the business requirement, by understanding the client business process flow.
Beyond that, critical thing is to categorize the requirement as Functional, Technical, Regulatory and Interface.
If requirement is Functional - Ask business justification
If requirement is Regulatory - Ask regulatory evidence
If requirement is Technical - Ask details with expected business volumes
If requirement is Interface - Ask details about both the system (A and B - where both integrates)