First of all, everyone including Krystal love to have fun. Who dont? Besides, she's only a teen for god's sake. Yes she said the basic isnt fun. Buts till she did it right? And look at her performance. If she didnt want to do it, she wont even practice in the first place. And in the end of the video, I dont sense any tense situation wbetween them at all. The atmosphere was harmonious and they treat each other like friends. Krystal even joked around with him.