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In my personal opinion the main difference between practice with old deities or entities and new ones is the amount of thought, emotion and belief that has gone into each.
Just like belief in general is a very personal thing. A deity will look different in every worshippers head.
Someone who encountered Thor through the Eddas will have a different picture of Him than someone who first encountered Him through, let's say Marvel comics, or the movies.
There is a theory that many deities originally started out as folk heroes, celebrated, and in following generations venerated, then worshipped.
We know the way a deity is depicted, how it is thought about, changes with time and location.
So, if enough people put enough energy into a fictional character,
that character can gain... how to say it... power/spiritual reality/momentum from it.
