Some years ago I did an illustration of Henry the Mouse and never quite got to illustration Matilda the cat. I was doing an art course at that stage that did not want any representational art in their curriculum that year.
However, I went to a pet shop to take photos of a small white mouse. I got my mouse character Henry from those photos and developed Henry the Mouse.
In a story every thing has to fit together and that is the hard part. The key is research and hard work by hand for hand done illustrations. Takes a while to do.