Usually, the illustrator comes up with the interview/brief because they have a specific workflow and would require you to answer specific questions that trigger their brain to complete the work the best way. However, that usually happens once you are committed as it takes time and they are already working. Some questions include main topic/themes, feelings, colors, storytelling elements and also a few images to actually show styles and details. I'd love to work with you if you're still looking!

Hi! I have a detailed blog post on this - https://www.illustrationsbysumitra.com/post/things-you-should-tell-your-children-s-book-illustrator