Regardless of whether you are writing an AU story or trying to adhere closely to Michael's reality, research is something that is necessary if you expect your story to have substance. While easily overlooked, research is what can take a story from "pretty good" to "this story is so real that I feel like I'm actually hearing Michael's thoughts!"
Some thoughts on how research can enhance your story:
Michael's speech. From the way he would often leave an "s" off of the end of "sometime" to idioms he'd use (e.g. "God bless you")
His family dynamics. This is crucial to creating a scene in which he interacts with different family members.
Where he was in a given month/year.
email@example.com is great for this.
His faith. Being a Jehovah's Witness played a huge role in his life and his development.
Not doing at least cursory research into that faith could mean missing a huge aspect of "Michael's" essence.
Reading other fan fics and/or fiction in general. See what you like, see what you don't. Try to tease out aspects of a story that draw you in and then make them your own.
NOTE: In saying that, I am not suggesting to take from the plot.
- Setting the scene. I'll never forget reading of a NYC in which cars could race down the streets at 80mph. Or someone driving from Santa Barbara to San Jose on a daily basis. Or Neverland being 30 minutes from LA.
These are all lapses in research. Did they break the stories? No, but they were jarring enough that I still remember them.
Stick to what you know (or can reasonably learn). Part of doing research is understanding what you don't know. Say, for instance, Michael is being charged with something and has to go to court. Look up the phrasing of such charges, the ordering of procedures, and/or the type of lawyer that would be appropriate. I've been asked to assist on everything from legal and criminal issues to expected behavioral patterns. I love it when an author asks for help from readers. Before a show goes on TV, it's shown to a selected group of viewers and their feedback is solicited to help shape the direction of the show. That doesn't mean the screen writers and actors give up their views, it just means they have more ideas to draw from in order to make the show a success.
Like watching Michael read musical notes, acting like a man from a middle class WASP family, getting details wrong kills the flow and leaves the reader thinking,
Often it's tempting to simply not do the research, because this is for fun, and saying to oneself, "I'm here to write my story, not read a bunch of stuff and make notes for junk, after all this is for fun!" But many stories have been ruined by not doing the research ahead of time. This may not be obvious to the writer, but it's often obvious to the reader who decides to stop reading without ever commenting.