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I suppose one alternative is to play it as you yourself found the problem: Start your detective out as a great detective but a novice occult detective, that just stumbled into the whole occult side...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/37041 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/37041 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
I suppose one alternative is to play it as you yourself found the problem: Start your detective out as a great detective but a novice _occult_ detective, that just stumbled into the whole occult side of things, and naturally knows nothing about it. He is certain it is really occult, due to some instigating incident early in the story, but the idea is new to him. As he investigates he finds this occult detective world is bigger than he imagined, there is competition, people he gets advice from, people that try to thwart him, and he hears crazy stories about Wiccan werebats and zombies. But he's a grumpy detective, stubborn, and he sets that aside for HIS quest, this one damn occult thing that has to be stopped, no matter what else is out there. In a way, he fits the role of "stranger in a strange land". You can take what you want from what is written, and write an original story _acknowledging_ the genre. Your character is vaguely aware the occult world is much deeper than he imagined starting out. Readers that DO have any expectations don't think you are uninformed, and it makes room for a sequel in which the next case is something just a little deeper. But instead of exploring this whole forest of genre, your character is yours, trudging a straight line _through_ the forest, seeing glimpses of other weird stuff, but this doesn't discourage him. He continues his single-minded pursuit of the one damned thing (literally) he knows must be neutralized.