I have had and sat in competency based. You're telling a story. It doesn't have to be entirely factual, base it around facts but make sure its full of content and shows you know how to and can do something even if you havent had the opportunity to yet. And its I, I did this, I helped with this. Even if it was your colleague or manager, I, I, I!!!
If they ask a question they usually want more content so give it. Did you have any feedback from that? Yes I did, I was contacted and told that my processes would be implemented in future guidance, which I've since heard are having 50% more positive results than they were previously.
Did the customer thank you? Yes he did (but add in that he sent an email of thanks to your manager who fed back to you how grateful he was). You can usually take notes in for competency based aswell.
Good luck