Hey guys, not sure if you've read it but I found this article form 2011 that has a TON of good insight into the making of Dan Santat's art! He gives a breakdown of his entire process on making Sidekicks, down to his 7 step illustration process and how he structures his Photoshop layers, and making a cat using John Goodman as reference.
I found this super helpful, hope you do too!
https://kathytemean.wordpress.com/2011/11/05/illustrator-saturday-dan-santat-2/
If you check out the "Illustrator Saturdays" section there are years of interviews like this with a bunch of other illustrators:
https://kathytemean.wordpress.com/category/illustrators-saturday/