Different IP owners have different policies regarding fan works. Most tolerate it -- sorta like "don't ask, don't tell" --  some are rabid persecutors of anything they even think is derived from/inspired by their characters -- Disney -- and a small few actually encourage it within certain limits like the "no porn (at least not in public or where kids can access it) and no profit" rule. So it's always best to check out which ones permit certain activities and which do not and abide by their terms.