Whenever possible, you should speak about your achievements from the perspective of an employer (helping them to find new customers, helping them to improve their reputation, building good atmosphere on the workplace, earning more money, etc), rather than achievements from your own perspective (getting promoted, earning a degree or certification, etc). You don't need to memorize an answer, but do take the time to consider how you'll respond. The more you prepare, the more confident you'll feel during a job interview. When you're not sure what to expect during an interview, also take time to review this refresher on how job interviews work, and these tips on how to prepare to ace your job interview.
What They Want to Know: What do you do when things don’t go smoothly at work? How do you deal with difficult situations? The employer wants to know how you handle workplace stress. Avoid claiming that you never, or rarely, experience stress. Rather, formulate your answer in a way that acknowledges workplace stress and explains how you’ve overcome it, or even used it to your advantage.
Any candidate can read and regurgitate the company’s “About” page. So, when interviewers ask this, they aren't necessarily trying to gauge whether you understand the mission—they want to know whether you care about it. Start with one line that shows you understand the company's goals, using a couple key words and phrases from the website, but then go on to make it personal. Say, “I’m personally drawn to this mission because…” or “I really believe in this approach because…” and share a personal example or two.