I'm going to have to disagree with you strongly that the relationship in Supernatural has not changed. It is between the same two people and they already had a relationship prior to the show, thus circumventing the normal "getting to know you" progression, but their relationship has changed substantially since season one, as have they as characters. It's not just circumstances that have changed. Supernatural, by putting the characters into untenable situations for their season-long arcs, also fosters a huge amount of fic that addresses those issues directly. (Last season, for example, there was a ton of fic about Dean's deal.) In that way the fic in the fandom is far more tied to canon than what I've seen of NCIS, which tends to more often use canon occurences as background for the character's emotional state rather than using them directly as plot points.
You would perhaps not be surprised to learn that slash fic in general often does not address the characters' relationships with and attitudes towards women. If it does, how the characters treat women is often used as subtextual evidence of the characters' bisexuality. And believe me, there are several metric tons worth of meta about how slash fic marginalizes women and how inherently problematic that is, so I don't really want to get into normative statements about this aspect of slash; I'm just saying that it is a feature of slash.
Though, in NCIS fandom, from what I've seen, since the show has quite a few strong and popular female characters, fic tends to treat them fairly well, Abby in particular. (Kate, on the other hand, seems to get the lion's share of the character bashing, at least in fic written prior to her death on the show.)
You are also absolutely right that the shows with more character interaction tend to be the shows with bigger fandoms. Hence NCIS and CSI being fairly large fandoms, and Bones also having a (much smaller) fandom, but the Law and Orders, though extremely similar in format, having almost no fandom at all. This isn't the only factor in which shows have fandoms, though, otherwise there would be no way to explain things like popslash, which essentially has no canon at all.
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You would perhaps not be surprised to learn that slash fic in general often does not address the characters' relationships with and attitudes towards women. If it does, how the characters treat women is often used as subtextual evidence of the characters' bisexuality. And believe me, there are several metric tons worth of meta about how slash fic marginalizes women and how inherently problematic that is, so I don't really want to get into normative statements about this aspect of slash; I'm just saying that it is a feature of slash.
Though, in NCIS fandom, from what I've seen, since the show has quite a few strong and popular female characters, fic tends to treat them fairly well, Abby in particular. (Kate, on the other hand, seems to get the lion's share of the character bashing, at least in fic written prior to her death on the show.)
You are also absolutely right that the shows with more character interaction tend to be the shows with bigger fandoms. Hence NCIS and CSI being fairly large fandoms, and Bones also having a (much smaller) fandom, but the Law and Orders, though extremely similar in format, having almost no fandom at all. This isn't the only factor in which shows have fandoms, though, otherwise there would be no way to explain things like popslash, which essentially has no canon at all.