Lead with user evidence, not design theory. "In our usability test, 4 out of 5 users missed this button" is more persuasive than "best practices say..."
Strong answers show evidence-based advocacy: using research data, competitive analysis, usability test results, or prototypes to make the case. The best candidates also show willingness to compromise when presented with valid counterarguments, and they maintain relationships even in disagreement.
Tests conviction and communication. Do they advocate with data or just opinions? Can they influence without authority? Do they know when to compromise?