Congrats! Mine was very casual, interviewer was super nice. Mostly just had a conversation but he did mention he had some questions from the adcom that he needed to fill out so I gave my answers to those (they were about specific ECs, classes, etc.). Really enjoyed speaking with him.
Writing exercise is writing a paragraph describing an image they assign you. I imagine it's for getting a writing sample of some sort. Other than that, there was another phone interview with a student a few days later, mostly just conversation with some questions here and there. Good opportunity to learn more about the school environment as a student.
Overall, really good vibes from both the interviewer and student!