Personality Question:

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You certainly don't want to be sitting there with your eyes to the ceiling and your mouth hanging open as you dig through your brain for an old experience to reference. You also don't want to say, "I can't think of any. The best thing you can do with these types of questions is to prepare for the interview by reviewing your work history. Go through your resume and letters of recommendations and any employment history and old communications you have.
Write down particularly interesting projects you worked on and think about positive experiences you can allude to.