What you said makes a lot of sense. i do think an "alternate universe" where some *episode* happens different is viable, but as we both say, the people still have to react to the situation the way the characters would.
I have written literally dozens of "FUll House" stories, as well as a number of other ones. I have remarked about staying in character, but not as much why. I've got almost no time to write anymore, but I thnk I have helped some to think, at least.
I think the whole "mary Sue" thing gets a bad rap - if a person whats a character to act like *they* want them to act, that's fine - it's only a Mary Sue/Marty Stu (for male ones) if the charactr is perfect or just happens to always come along at the right time. Otherwise, it's perfectly legitimate to include one's own characters, and have the characters int he fandom react to you.
ise. (
A perfect example if the Sam Series done by my good friend RKORadio on fanfiction.net - he created a very realistic neglected girl who the middle Tanner child, Stephanie, wound up helping even as a kid, and who developed into someone who saw Steph as her mother figure. The character development is xcellent, and lots of fun. He picked out the character most likely to be able to help and bond with this troubled child, and crafted an entire series around her. Actually tied in some Full Hoise. (use books so they made more sense than they might have otherwise. (I don't know of any 12-year-old who could ever be given use of her dad's credit cards, like Steph is in one book. *However*, if she's handled some things with an incredible amount of responsibility, than he might..)
Sorry, I'm rambling now; I invite anyone who is interested to check out RKORadio's work at fanfiction.net. It's a great example of inventing one's own characters.