Maybe I'm just stating the obvious here. To an illustrator maybe this is obvious. Is it?
Any illustrator of sequential art--picture books, mid-grade novels, comics, graphic novels, etc.--should always start by pitching, and getting approval on, character sketches for said project. Even if they're not asked for (in my experience, not always), and even if it feels so right that you just wanna dive into the thing and start sketching pages. Personally speaking, in my humblest opinion, it's best to get the look of the character or characters down relatively tight, and agreed upon by both parties, before attempting to do the rest of the work.
I'm working on two middle-grade (3rd-4th-5th grade) novels right now that are asking for about 50 drawings each. The first book's sketch deadline was right around the time Romy was born. Needless to say, at that time my brain was fried and probably re-fried. Probably, I spaced. I did not show character sketches on that book. Instead, cranked on 55-ish sketches and sent them in. And... Ehhh... Ohhh... Didn't come back so well. Could've been a LOT worse, but the point is, I went right in without a lead. I did some character sketches but did not share. I used those to get going and sketched all the pages. Today, I'm currently near approval on a set of character sketches for this book, and pretty much back on track.
Needless to say, I was quickly reminded before the second book's sketch deadline (soon!) of my old rule of thumb. Gots to show them character sketches!
And, er, what're character sketches you might ask. A group of sketches (full-body is best, plus maybe some head shots for face details?) that show each of the main characters I'd be working with on a book. Sometimes I like to show each character from a couple of different angles. Either way, each sketch should show the personality and look I'm going for--of the person/animal/thing I'll be drawing many times over.
Here is a group of character sketches for book 2, in various states of approval.