My interview was incredibly informal. It was more like a conversation about what I wanted to do as a journalist and why I wanted to attend Berkeley. Just be relaxed and have fun with it.
As for the writing test, make sure you're caught up on the big news over the last year. You'll be asked to give written responses to two questions in 30 mins. I think everyone has a different way of handling the time crunch, but make sure you leave a minute for each question to do editing.
Good luck!