I was already a fan of the book series before the show came out - I forget how many of the books were out at that point, but I'd definitely read the first three at least.

So I came at it the other way around, especially with Amos. I remember when Wes Chatham was first announced as Amos, and I thought he looked entirely wrong for the role. But he completely blew me away with his portrayal. He brought so much depth and nuance to Amos, and made me utterly fall in love with a character that I'd previously been kind of 'meh' about. Wes is now my definitive Amos, and I just kind of mentally gloss over the book descriptions that contradict that. He's my favourite character now, though Camina Drummer (the show version) comes a very close second.

I didn't have the same trouble with Fred Johnson though. Neither version was a favourite or 'meh' for me.