I find I've been posting a story on Raccoon's Bookshelf where should have (formally) asked permission from authors of characters I use.
That is bad form, since Max Blackrabbit certainly replies on such requests.

The issue is a little less obvious for authors such as EWS. He's known to disregard such requests, he simply out he doesn't want you to make money on his creations.
Same for the author of the ZZ Studios stories, James Bruner, but he requests one to ask for permission.
The author who gave me a broad OK was Chris Foxx, that leaves: EWS (how?), Max (I think I mess that up) and specifically James Bruner.
Yes, it gets really convoluted if you use Zig Zag, James, Doug, Kelly, Tabitha and Richard in one story. I for one won't ever try that again, and I am hoping to get one or two OKs.