I think one of the easiest things to get wrong about depicting other people's belief systems is how to predict how much they will follow what they say they will follow. When you're talking about someone from the same larger cultural umbrella as yourself, it's much easier: I have a sense, for example, of how much my Catholic friends and neighbors will change their mind on an issue if the Pope issues a new ex cathedra statement about it. I know what the range of responses will be for Midwestern American Lutherans if the ELCA comes out with a resolution, or if their own pastor preaches a sermon. But I find this much harder to predict with a religion or other belief system I'm less familiar with, and I think it's simultaneously disrespectful to assume that members of group X will do exactly what their holy book, leader, etc. tell them to because clearly they don't have minds of their own, or to assume that members of group X will ignore their holy book, leader, etc., because clearly they don't take that stuff seriously.
And that sort of stated or implied reasoning is big part of the problem. I know that a lot of my Catholic friends and neighbors disagree with the Pope because they're devout and believe their interpretation of human sexuality or other human rights issues is a better expression of Catholicism than his. But if I don't know a lot of, say, Japanese animists, and I come up with something that is supposedly a deeper expression of their animism than the stated beliefs, then I have fallen into the smelly pit known as What These People Need Is A Honky. (Or a Gentile. Or just My Brilliant Self; this is not limited to non-white groups.) I have been in arguments with people who decided they knew what I thought and believed better than I did, and it was condescending and unpleasant. I don't care to be that person.
Which doesn't mean I'm off the hook for depicting people from more distant/different belief systems--and on the up side, figuring out the details of this sort of thing is one of the more interesting parts of making up a belief system. I've just been thinking of this as one of the hard bits to remind myself so I don't default to behaving as though fake fantasy religions are all approximately Midwestern American Protestantism in funny hats.
This was not part of my original comment, but I wonder if this kind of mismatch in how much to assume people "mean it" and which directions of "meaning it" they will follow is part of the disconnect in how we talk about politics and religion in this country. Various subcultures have different assumptions about what to take literally that don't always match up very well.