Fanfiction is not written based on original characters. MJ fanfiction is written based on Michael, so it stands to reason that getting Michael "right" is a fundamental task for any writer of MJ fanfic. Thus, regardless of whether you are writing a very AU story, readers will constantly compare your take on Michael to what they know of him. Thus, many will be livid if you turn him into a murderer, a rapist, and child molester....or even a ridiculously innocent naive adult. For this reason, you shouldn’t go changing much about his personality unless you’re going to write a disclaimer and commit to that alteration 100%.
Know the man
In Depth: If you are going to write canon (or something very similar to it), do your research. I am not joking. Do it. If you don't want people like me butting in, finding yourself flustered, or just plain overwhelmed, then do your research. From the procedures he had done to the nicknames he used to the people he avoided to the volume of his voice in certain situations to his drinks of choice to the foods he actually regularly ate in different eras. Many of us readers would like to help and will help if/when asked. At the same time, we can't be your sole source of information.
Think about it this way, how often was Michael portrayed in a false light by the media? By people who didn't like him? Even by fans, themselves? Enough that he had to develop "elephant skin". Do you want to be part of that group that perpetuated falsehoods? If not, do your homework. If you do not want to do true canon, add in an author's note at the beginning of your story denoting the fact that you are taking some creative liberties, specifying in which ways you are. If you can give a reasonable explanation of why you changed his personality, most readers will forgive you.
Know what you don't know
Summary: None of us are Michael, so all of us have at least some significant gaps in knowledge. Know yours.
In Depth: if you ever find yourself guessing or making something up, realize that that is what you are doing. If you are writing canon, these instances should be as minimal as possible. How do you accomplish this? Research. Get a sense of Michael's personality so that you can make a reasonable guess as to what he might say or do in a given situation.
Stop the glorification ( or the infantilization)
Summary: Michael was The Man. That doesn't make him perfect. Mmmkay?
In Depth: He was human. He wasn't always naive and innocent. He also wasn't always perfect. He sometimes made off color jokes to fans. He also sometimes swore. He drank during different times of his life...and sometimes got wasted. He sometimes avoided his siblings and he sometimes invited fans into his house. He wasn't always sequestered or always reachable. Shoot, he pursued at least one married woman. He was human. Let that shine through.
Write his Bio (first!)
Summary: Reread my last few points. Now use them to write his bio.
In Depth: And by "first!", I mean, before you start writing him in your story. Then, whenever you have a question about his dialogue or something he does, refer back to that bio. Is it consistent?
Read what you write out loud
Summary: Read some of the dialogue out loud to make sure that each character doesn't sound the same.
In Depth: People use different phrasing, disclose different amounts of information, and have different speech patterns. This isn't all distinguished by simply giving the OG and Michael different types of pet names to say. Reading snippets of the dialogue aloud can help you hear if they sound different without having to change your tone of voice. When you read out loud you can actually hear how bad things sound. Some people use bigger words, others use more slang, others speak different types of slang.
Listen to your feedback
Summary: Sometimes we all slip up. Sometimes someone else knows more. Listen, question, and reconsider if the feedback is valid and/or appropriate.
In Depth: Readers in general tend to tip toe around what they don't like, keep comments short, and just encourage. When someone actually points out a canon/near-canon Michael acting out of character in your fanfic, then I'd suggest going back and looking over the chapter/story again to see if a fix needs to happen (whether by actually editing the chapter or addressing it in an upcoming chapter). Since you can interact with readers in the comments, you might even ask them questions if you are unclear how it is OOC. Now, if you don't agree with some things that's perfectly fine, but do take into account that their may be other readers who have the same thoughts as the person who spoke up. If nothing else, file this in the back of your mind under "things to consider" as you move forward in your story.