When Chad is looking for illustrators, he's looking for a unique voice. And one of the best exercises he's found is to make yourself do some fan art of a favorite work, for example designing YOUR version of the Harry Potter cover(s). To Chad, style is just the straight look of your work, it doesn't mean 'watercolor' or 'digital', your brand is your stamp on the work. You can evolve that brand over time but Chad is looking for consistency within those iterations.
Even with fan art you should be thinking about all of the different angles of the package, in this case a hardcover book. How do the cover image, title type, spine, flaps and endpapers all work together? Your brand as an illustrator should be evident on the book as a whole.
When Chad is looking to take on a new client he asks himself what kind of work can he get them—board books, YA covers, etc.?
Chad has his illustrators look through their own online portfolios and Instagram posts and ask themselves if the pieces they are showing look like the work they want to be getting and is 'on brand'.
Every single element can be branded, these thoughtful banner examples below make it clear in one image the voice and style of the illustrator, the subjects that are important to them, and a clear indication of where their work fits in to the children's book industry (board books, comics, YA covers, etc.):
How do you build your voice? Chad says to focus on you and what is enjoyable to you. He has his illustrators make a list of 100 things that they love to draw (from John Hendrix's book DRAWING IS MAGIC) and then to find a way to include some of those things in all of your drawings, always.