This question is often used to assess how well you perform under pressure as well as your problem-solving abilities. Keep in mind stories are more memorable than facts and figures so strive to ‘show’ instead of ‘tell’. This is also an excellent opportunity to show your human side and how you’re willing to go the extra mile without being asked.
Example: “It was the first day of my boss’s two-week vacation and our agency’s highest-paying client threatened to leave because he didn’t feel he was getting the personalised service he was promised. I spent my lunch hour on the phone with him talking through his concerns. We even brainstormed ideas for his next campaign. He was so grateful for the personal attention that he signed another six-month contract before my boss even returned from her trip.”