The most important thing to do when you first work on a creative brief is RESEARCH. Don’t rely on planners to give you all the information you need; do your own research. Often the best insights are in the smallest of details. By getting to know the subject as well as possible and immersing yourself in the product, you are enriching your understanding of how something works. Little things other people assume are insignificant could be golden nuggets waiting to be formed into a great idea.
(Further reading: Interesting article related to the importance of research in advertising.)