Each property has different rules; fortunately, Warner Bros are pretty permissive about Potter fan fiction. One of the obvious rules is “no profit-making”, so we’re not going to go on revenue-generating channels like Seeka. At this point, there are so many other fan film projects that use the original characters, unlike us, so we’re well below the radar where we’d get a letter from a lawyer.
The plus side is that there’s a built-in, highly enthusiastic fan base. We’ve screened and/or gotten vendor tables at general fan cons, as well as Potter-specific ones. Before the Fantastic Beasts series started, people were eager for any new Potter-related content, which helped us out a lot.